Precursor coating compositions containing water and an organic coupling solvent suitable for spraying with supercritical fluids as diluents

ABSTRACT

The present invention relates to precursor coating compositions containing water and at least one organic solvent which are particularly suitable for being admixed with at least one supercritical fluid used as a viscosity reduction diluent and then spraying this resultant liquid mixture of supercritical fluid and precursor coating composition onto a substrate to be coated. Processes for spraying this precursor mixture are also disclosed.

This application is a continuation of prior U.S. application Ser. No.327,274 filed on Mar. 22, 1989, now abandoned.

RELATED PATENT APPLICATIONS

This application contains subject matter related to application Ser. No.133,068, filed Dec. 21, 1987, which application is acontinuation-in-part of application Ser. No. 883,156, filed Jul. 8,1986, now abandoned. This application also contains subject matterrelated to U.S. patent applications Ser. No. 218,896, filed Jul. 14,1988; and Ser. No. 218,910, filed Jul. 14, 1988.

FIELD OF THE INVENTION

This invention, in general, pertains to the field of coatingcompositions. More specifically, the present invention relates toprecursor coating compositions, containing water and at least oneorganic coupling solvent, which are particularly suitable for beingadmixed with at least one supercritical fluid used as a viscosityreduction diluent. The resultant admixed liquid mixture of supercriticalfluid and precursor coating composition can then be sprayed onto asubstrate to be coated.

BACKGROUND OF THE INVENTION

Prior to the present invention, the liquid spray application ofcoatings, such as lacquers, enamels and varnishes, was effected solelythrough the use of organic solvents as viscosity reduction diluents.However, because of increased environmental concern, efforts have beendirected to reducing the pollution resulting from painting and finishingoperations. For this reason there has been a great deal of emphasisplaced on the development of new coatings technologies which diminishthe emission of organic solvent vapors. A number of technologies haveemerged as having met most but not all of the performance andapplication requirements, and at the same time meeting emissionrequirements and regulations. They are: (a) powder coatings, (b)water-borne dispersions, (c) water-borne solutions, (d) non-aqueousdispersions, and (e) high solids coatings. Each of these technologieshas been employed in certain applications and each has found a niche ina particular industry. However, at the present time, none has providedthe performance and application properties that were initially expected.

Powder coatings, for example, while providing ultra low emission oforganic vapors, are characterized by poor gloss or good gloss with heavyorange peel, poor distinctness of image gloss (DOI), and poor filmuniformity. Moreover, to obtain even these limited performanceproperties generally requires excessive film thicknesses and/or highcuring temperatures. Pigmentation of powder coatings is often difficult,requiring at times milling and extrusion of the polymer-pigmentcomposite mixture followed by cryogenic grinding. In addition, changingcolors of the coating often requires its complete cleaning, because ofdust contamination of the application equipment and finishing area.

Water-borne coatings are very difficult to apply under conditions ofhigh relative humidity without serious coating defects. These defectsresult from the fact that under conditions of high humidity, waterevaporates more slowly than the organic cosolvents of the coalescingaid, and as might be expected in the case of aqueous dispersions, theloss of the organic cosolvent/coalescing aid interferes with filmformation. Poor gloss, poor uniformity, and pin holes unfortunatelyoften result. Additionally, water-borne coatings are not as resistant tocorrosive environments as are the more conventional solvent bornecoatings.

Coatings applied with organic solvents at high solids levels avoid manyof the pitfalls of powder and water-borne coatings. However, in thesesystems the molecular weight of the polymer has been decreased andreactive functionality has been incorporated therein so that furtherpolymerization and crosslinking can take place after the coating hasbeen applied. It has been hoped that this type of coating will meet theever-increasing regulatory requirements and yet meet the most exactingcoatings performance demands. However, there is a limit as to theability of this technology to meet the performance requirement of acommercial coating operation. Present high solids systems havedifficulty in application to vertical surfaces without running andsagging of the coating. Often, they are also prone to cratering and pinholing of the coating. If they possess good reactivity, they often havepoor shelf and pot life. However, if they have adequate shelf stability,they cure and/or crosslink slowly or require high temperature to effectan adequate coating on the substrate.

Clearly, what is needed is an environmentally safe, non-pollutingdiluent that can be used to thin very highly viscous polymer andcoatings compositions to liquid spray application consistency. Such adiluent would allow utilization of the best aspects of organic solventborne coatings applications and performance while reducing theenvironmental concerns to an acceptable level. Such a coating systemcould meet the requirements of shop- and field-applied liquid spraycoatings as well as factory-applied finishes and still be in compliancewith environmental regulations.

Such a needed diluent has now been found and is discussed in theaforementioned related applications which teach, among other things, theutilization of supercritical fluids, such as supercritical carbondioxide fluid, as diluents in highly viscous organic solvent borneand/or highly viscous non-aqueous dispersions coatings compositions todilute these compositions to application viscosity required for liquidspray techniques.

U.S. patent application Ser. No. 133,068, filed Dec. 21, 1987, to Hoy,et al., disclose processes and apparatus for the liquid sprayapplication of coatings to a substrate that minimize the use ofenvironmentally undesirable organic diluents. The broadest processembodiment of that application involves:

(1) forming a liquid mixture in a closed system, said liquid mixturecomprising:

(a) at least one polymeric compound capable of forming a coating on asubstrate; and

(b) at least one supercritical fluid, in at least an amount which whenadded to (a) is sufficient to render the viscosity of said mixture of(a) and (b) to a point suitable for spray application; and

(2) spraying said liquid mixture onto a substrate to form a liquidcoating thereon.

That application is also directed to a liquid spray process in which atleast one active organic solvent (c) is admixed with (a) and (b) aboveprior to the liquid spray application of the resulting mixture to asubstrate. The preferred supercritical fluid is supercritical carbondioxide. The process employs an apparatus in which the mixture of thecomponents of the liquid spray mixture can be blended and sprayed ontoan appropriate substrate. The apparatus contains

(1) means for supplying at least one polymeric compound capable offorming a continuous, adherent coating;

(2) means for supplying at least one active organic solvent;

(3) means for supplying supercritical carbon dioxide fluid;

(4) means for forming a liquid mixture of components supplied from(1)-(3); and

(5) means for spraying said liquid mixture onto a substrate.

The apparatus may also provide for (6) means for heating any of saidcomponents and/or said liquid mixture of components. U.S. patentapplication Ser. No. 133,068 demonstrates the use of supercriticalfluids, such as supercritical carbon dioxide fluid, as diluents inhighly viscous organic solvent borne and/or highly viscous non-aqueousdispersions coatings compositions to dilute the compositions toapplication viscosity required for liquid spray techniques. It furtherdemonstrates that the method is generally applicable to all organicsolvent-borne coatings systems.

Copending U.S. application Ser. No. 218,910, filed Jul. 14, 1988, isdirected to a liquid coatings application process and apparatus in whichsupercritical fluids, such as supercritical carbon dioxide fluid, areused to reduce to application consistency, viscous coatings compositionsto allow for their application as liquid sprays. The coatingscompositions are sprayed by passing the composition under pressurethrough an orifice into the environment of the substrate.

In particular, the process of U.S. application Ser. No. 218,910 forliquid spray application of coatings to a substrate comprises:

(1) forming a liquid mixture in a closed system, said liquid mixturecomprising:

(a) at least one polymeric component capable of forming a coating on asubstrate; and

(b) a solvent component containing at least one supercritical fluid, inat least an amount which when added to (a) is sufficient to render theviscosity of said mixture to a point suitable for spray application; and

(2) spraying said liquid mixture onto a substrate to form a liquidcoating thereon by passing the mixture under pressure through an orificeinto the environment of the substrate to form a liquid spray.

U.S. application Ser. No. 218,895, filed Jul. 14, 1988, is directed to aprocess and apparatus for coating substrates by a liquid spray inwhich 1) supercritical fluid, such as supercritical carbon dioxidefluid, is used as a viscosity reduction diluent for coatingformulations, 2) the mixture of supercritical fluid and coatingformulation is passed under pressure through an orifice into theenvironment of the substrate to form the liquid spray, and 3) the liquidspray is electrically charged by a high electrical voltage relative tothe substrate.

In particular, the process of U.S. application Ser. No. 218,895 forelectrostatic liquid spray application of coatings to a substratecomprises:

(1) forming a liquid mixture in a closed system, said liquid mixturecomprising:

(a) at least one polymeric component capable of forming a coating on asubstrate; and

(b) a solvent component containing at least one supercritical fluid, inat least an amount which when added to (a) is sufficient to render theviscosity of said mixture to a point suitable for spray application;

(2) spraying said liquid mixture onto a substrate to form a liquidcoating thereon by passing the mixture under pressure through an orificeinto the environment of the substrate to form a liquid spray; and

(3) electrically charging said liquid spray by a high electrical voltagerelative to the substrate and electric current.

The use of supercritical fluids as a transport medium for themanufacture of surface coatings is well known. German patent application28 53 066 describes the use of a gas in the supercritical state as thefluid medium containing the solid or liquid coating substance in thedissolved form. In particular, the application addresses the coating ofporous bodies with a protectant or a reactive or nonreactive decorativefinish by immersion of the porous body in the supercritical fluidcoupled with a pressure drop to effect the coating. The most significantporous bodies are porous catalysts. However, the applicant characterizesfabrics as porous bodies.

Smith, U.S. Pat. No. 4,582,731, patented Apr. 15, 1986, and U.S. Pat.No. 4,734,451, patented Mar. 29, 1988, describe forming a supercriticalsolution which includes a supercritical fluid solvent and a dissolvedsolute of a solid material and spraying the solution to produce a"molecular spray." A "molecular spray" is defined as a spray "ofindividual molecules (atoms) or very small clusters of the solute." TheSmith patents are directed to producing fine films and powders. Thefilms are used as surface coatings.

Coating formulations are commonly applied to a substrate by passing thecoating formulation under pressure through an orifice into air in orderto form a liquid spray, which impacts the substrate and forms a liquidcoating. In the coatings industry, three types of orifice sprays arecommonly used; namely, air spray, airless spray, and air-assistedairless spray.

Air spray uses compressed air to break up the liquid coating formulationinto droplets and to propel the droplets to the substrate. The mostcommon type of air nozzle mixes the coating formulation andhigh-velocity air outside of the nozzle to cause atomization. Auxiliaryair streams are used to modify the shape of the spray. The coatingformulation flows through the liquid orifice in the spray nozzle withrelatively little pressure drop. Siphon or pressure feed, usually atpressures less than 18 psi, are used, depending upon the viscosity andquantity of coating formulation to be sprayed.

Airless spray uses a high pressure drop across the orifice to propel thecoating formulation through the orifice at high velocity. Upon exitingthe orifice, the high-velocity liquid breaks up into droplets anddisperses into the air to form a liquid spray. Sufficient momentumremains after atomization to carry the droplets to the substrate. Thespray tip is contoured to modify the shape of the liquid spray, which isusually a round or elliptical cone or a flat fan. Turbulence promotersare sometimes inserted into the spray nozzle to aid atomization. Spraypressures typically range from 700 to 5000 psi. The pressure requiredincreases with fluid viscosity.

Air-assisted airless spray combines features of air spray and airlessspray. It uses both compressed air and high pressure drop across theorifice to atomize the coating formulation and to shape the liquidspray, typically under milder conditions than each type of atomizationis generated by itself. Generally the compressed air pressure and theair flow rate are lower than for air spray. Generally the liquidpressure drop is lower than for airless spray, but higher than for airspray. Liquid spray pressures typically range from 200 to 800 psi. Thepressure required increases with fluid viscosity.

Air spray, airless spray, and air-assisted airless spray can also beused with the liquid coating formulation heated or with the air heatedor with both heated. Heating reduces the viscosity of the liquid coatingformulation and aids atomization.

In general, coating compositions are formulated to help minimize thecoating defects that may occur after the coating composition has beensprayed by any of the above means onto the substrate and then dried.Such defects include, but are certainly not limited to, orange peelconditions, runs or sags, pin holing and solvent pops, fish eyes,blistering, and the like, all of which are well known to those skilledin this art.

Indeed, some coating formulations are provided in concentrated form,that is, with a relatively high solids content, so that they may becustom tailored on site by the user. Thus, the user adds an appropriateamount of a particular solvent mixture to accommodate a particular enduse spraying condition which may include variable wind conditions,ambient temperatures, drying conditions, humidity, and other suchspraying condition factors.

While the above-noted related patent applications all utilizesupercritical fluids as a diluent to help reduce the viscosity of highlyviscous organic solvent-borne and/or highly viscous non-aqueousdispersions coating compositions so as to facilitate the application ofthese compositions by liquid spray techniques, and by doing so,desirably reduce the amount of organic solvent which would otherwise beused, the overall objective, of course, is still to obtain a coatedsubstrate having a uniform, smooth, continuous coating and substantiallynone of the above-noted defects.

For obvious reasons, none of the prior art coating compositions havebeen formulated with the intent of having these compositions combinedwith a supercritical fluid as a diluent and then spraying the resultantadmixed liquid mixture through an orifice and onto a substrate to form aliquid coating which is then dried and/or cured.

Indeed, prior to the inventions described in the above-noted relatedapplications and the present invention, it was unknown how a highconcentration of highly volatile supercritical fluid, such assupercritical carbon dioxide fluid, would affect formation of a liquidspray containing a solids fraction; a diluent fraction in which saidsolids fraction is dissolved, suspended or dispersed, and a portion ofthe supercritical fluid. A spray mixture undergoes a large and rapiddrop in pressure as it goes through the orifice. Accordingly, one ofordinary skill in the art could theorize that the supercritical spraymixture would produce a foam like shaving cream instead of a spray,because nucleation to form gas bubbles would be so rapid and intense.Alternatively, one of ordinary skill in the art could also expect thatthe spray mixture would produce a mist or fog of microdroplets insteadof a spray, because atomization would be so intense. Another result thatone could theorize is that the spray mixture would produce a spray ofbubbles instead of droplets. Furthermore, even if a spray were formed,it would have been expected that the sudden and intense cooling thataccompanies rapid depressurization and expansion of a supercriticalfluid would cause the liquid droplets to freeze solid. For example, itis commonly known that the spray from carbon dioxide fire extinguishersproduces solid dry ice particles.

In the event that formation of a liquid spray were achieved, there is noassurance that the spray could be used to produce quality coherentpolymeric coatings on a substrate. One of ordinary skill in the artcould surmise that the liquid droplets would be so small or have solittle momentum that they could not be deposited well onto thesubstrate. One could also theorize that foaming droplets orsupercritical fluid dissolved in the coating would produce a layer offoam on the substrate or a coating full of bubbles when thesecharacteristics were not desired in the coating. The liquid coatingdroplets that are deposited onto the substrate would have a much higherviscosity than the material that was sprayed, because they would havelost most of the supercritical fluid diluent and they would be at alower temperature. Furthermore, the coating material would contain lessvolatile organic solvent than normal. Therefore, it is not unreasonableto expect that higher viscosity would prevent or hinder coalescence ofthe deposited droplets to form a coherent liquid coating; that it wouldreduce how much the droplets spread out on the substrate, so that thincoatings could not be produced; and that it would reduce the surfaceflow that produces a smooth coating. One can further theorize thatmoisture would condense onto the droplets and harm the coating, becausethe spray would be cooled below the dew point.

Surprisingly, however, it has been shown, as discussed in applicationSer. No. 883,156 noted above, that liquid sprays can indeed be formed byusing supercritical fluids as viscosity reduction diluents and that suchsprays can be used to deposit quality coherent polymeric coatings ontosubstrates.

However, after admixing the highly viscous organic solvent borne and/orhighly viscous non-aqueous dispersions coating compositions withsupercritical fluids as a diluent to help reduce the viscosity, it maystill be desirable to reduce the viscosity even further but keep theoverall amount of supercritical fluid used substantially the same.Alternatively, it may also be desirable to maintain (or lower) theviscosity of the admixed coating composition and maintain the overallamount of supercritical fluids used substantially the same, but stillwant to reduce even further the amount of organic solvent in the admixedcoating composition.

More specifically, there may be coating compositions whose initialviscosity is so high that the amount of supercritical fluids that can beadmixed with such compositions, without undesirably causing a two phaseseparation, is insufficient to reduce the viscosity to the point wheresuch composition can properly be sprayed.

Alternatively, since it is known that high molecular weight polymersgenerally provide finished coatings having better exterior durability,toughness, strength and solvent resistance, it may be desirable to usesuch a high molecular weight polymer in a coating composition in lieu ofa similar but lower molecular weight polymer that may be there. However,the use of such a high molecular weight polymer introduces an increasein the overall viscosity of the coating composition. This increase inviscosity may be such that the amount of supercritical fluids now neededto reduce the viscosity of the composition to a point suitable for sprayapplication may not be obtainable without breaking up the compositioninto two phases.

Still further, for a given highly viscous coating composition containinga particular amount of polymeric component and an organic or non-aqueoussolvent, respectively, it may be desirable to reduce the amount of suchvolatile solvents even further. Of course, such a reduction in solventwould inherently result in a corresponding increase in the overallviscosity of the coating composition. Here again, the increase inviscosity may be such that the amount of supercritical fluids needed tonow reduce the viscosity of the composition to a point suitable forspray application may not be obtainable.

Clearly, a need exists to be able to accomplish all of the aboveobjectives. Preferably, these objectives should be able to be carriedout without the necessity of adding supercritical fluid in an amountwhich is greater than that originally utilized, such that the expecteddiluent effect of the supercritical fluids can be expected to remainsubstantially about the same. Of course, if desired, more than theoriginal amount of supercritical fluid may be used, if such amount doesnot cause the excessive breakup of the composition into two phases.

Accordingly, the present invention provides a means by which the abovenoted goals may indeed be achieved and, more particularly, providesprecursor coating compositions in which those goals have beenmanifested.

Moreover, a need also exists to provide precursor coating compositionswhich in addition to achieving the above objectives are also formulatedto:

(a) be particularly compatible for subsequent admixture with asupercritical fluid diluent;

(b) be particularly suitable, once admixed with the supercritical fluid,to help minimize any of the phenomena that may occur which arepeculiarly associated with the utilization of such supercritical fluid,which phenomena may interfere with proper atomization of the admixedliquid mixture and/or proper diffusion of the supercritical fluid onceatomized; and

(c) provide the necessary coating characteristics such that once sprayedonto a substrate, it will help provide the necessary coalescence of thedeposited droplets to form a coherent liquid coating while still notcausing sagging or runs and help minimize any of the other defects notedabove while at the same time, still allow for the release of anyresidual supercritical fluid that may be present after the coating hasbeen applied to the substrate.

Accordingly, the present invention provides such precursor coatingcompositions which not only fulfill the goals of (1) having an evenlower viscosity and/or (2) having even less organic solvent, but whichare also particularly suitable for subsequent admixture with at leastone supercritical fluid which admixture is then sprayed through anorifice, such as airless spray or air-assisted airless spray methods, toapply an admixed coating composition onto a substrate which results in asubstrate having a substantially uniform, continuous and substantiallydefect-free coating.

SUMMARY OF THE INVENTION

In the more broader aspects of the present invention, it hasunexpectedly been found that water may actually be added to an organicsolvent-borne coating composition such that when admixed withsupercritical fluids, the water acts as an additional viscosityreduction diluent providing a composition having an even lowerviscosity. Most importantly, however, the amount of supercritical fluidsthat are miscible with this water-containing coating composition remainsat least substantially the same as in the composition in which no wateris present.

This discovery is quite surprising in that it has been found thatmaterials such as liquid carbon dioxide or supercritical carbon dioxideare only sparingly miscible with water or water-borne polymer mixtures.Yet, when in the presence of at least one organic coupling solvent,quite surprisingly, a relatively large amount of water may be added tothe organic solvent-borne coating composition under supercriticalconditions while still retaining the supercritical fluid miscibilitycharacteristics of the original composition. In general, up to about 30percent by weight of water, based on the total weight of solvent/diluentpresent in the composition, may be added with substantially no reductionin the amount of supercritical fluid contained in the composition.

Accordingly, in the illustration noted earlier in which not enoughsupercritical fluid could be added to a viscous coating composition soas to reduce its viscosity to a point suitable for spraying, thisproblem can now be solved by simply adding enough water to thecomposition (up to about 30 percent by weight of the totalsolvent/diluent present), so as to reduce the initial viscosity of thecomposition, while still keeping the amount of supercritical fluid thatis capable of being admixed with the composition the same. In otherwords, the addition of the water to the composition serves to act as afurther diluent to reduce the viscosity of the composition but does notsubstantially reduce the miscibility of the now water-containingcomposition with the supercritical fluids. Most importantly, such aviscosity reduction is achieved without adding organic solvent over andabove that which was originally present. While a coupling solvent isdesirably added to the composition in conjunction with such wateraddition, as will be more fully discussed hereinbelow, such couplingsolvent may be used to replace some or all of the organic solventpresent in the original composition such that the total amount oforganic solvent in the water-containing composition is less than orequal to the amount contained in the original composition. With such aviscosity reduction in the new water-containing composition, the amountof supercritical fluids that can be admixed with this composition isgenerally enough to reduce the viscosity further to a point suitable forspraying.

Similarly, in the illustration noted above in which it would bedesirable to replace a low molecular weight polymer with a similarpolymer having a higher molecular weight, but the amount ofsupercritical fluids that can be added to the new formulation cannot beincreased to compensate for the increase in viscosity, that too can nowbe accomplished by adding water to the system. The water acts as afurther diluent, and in conjunction with the supercritical fluids (thetotal amount of which remains substantially the same in both theoriginal composition and in the composition containing water), theviscosity of the reformulated composition containing the highermolecular weight polymer is now reduced to the point that the amount ofsupercritical fluids that can be admixed with the composition is nowenough to reduce the viscosity to a point at which it can be sprayed.

Most significantly, in contrast to the above two illustrations in whichwater is typically added to a composition so as to actually increase theoverall amount of solvent/diluent that is present, the present inventionhas also recognized that water may also be used to actually replace someof the organic solvent in the original composition. In this manner,while keeping the overall amount of solvent/diluent in the compositionsubstantially about the same, it is possible to reduce even further theamount of volatile organic or non-aqueous solvent that is present in thecoating composition so as to accommodate, if needed, the everincreasingly stringent guidelines that are being imposed.

Of course, the present invention recognizes that it is not necessary tostart with one composition formulation and then modify it by theaddition of water. The present invention clearly encompasses theformulation of an initial composition which is formulated with water inaccordance with the present invention.

Still further, the present invention is also directed to coatingcompositions which are intended for admixture with at least onesupercritical fluid and then subsequently sprayed onto a substrate as aliquid coating, particularly coating compositions containing water andorganic solvent in accordance with the present invention, and formulatedso as to provide such coating compositions with physical and/or chemicalcharacteristics which make them eminently suitable for such intendeduse.

In particular, by optimizing one or more specific physical and/orchemical properties of the coating composition, a number of factorsinfluencing and affecting the overall coating process, which includesthe utilization of a supercritical fluid, are significantly improved.Such factors include, but are not limited to, for example, (1) the easewith which the supercritical fluid is admixed with such compositions;(2) the amount of supercritical fluid that is capable of being admixedwith the composition while still desirably maintaining a single phase;(3) the ease with which the resulting liquid admixture is sprayed; (4)the quality of the atomized spray droplets that are produced; (5) theability of the supercritical fluid to rapidly diffuse from the atomizedsprayed droplets; (6) the ability of the atomized liquid droplets to bedeposited efficiently onto the substrate; (7) the ability of theatomized liquid droplets, once applied to the substrate, to sufficientlycoalesce and form a coherent liquid coating; (8) the ability for anyresidual supercritical fluid still remaining in the coating applied tothe substrate to effectively diffuse and escape; (9) the ability to helpform an essentially defect free coating; and the like, all of which areaffected, at least in part, by the characteristics of the precursorcoating composition.

It is recognized that variables other then the coating composition perse may have an effect on some or all of the above noted factors. Forexample, the spray temperature, the spray pressure, the particularsupercritical fluid being used, the amount of supercritical fluidadmixed with the precursor coating composition, the temperature andpressure of the environment in which the substrate is present, thedistance between the spray orifice and the substrate, and the like, allhave an effect upon the spraying process and the coating results thatare obtained. Generally, however, assuming that all of such processvariables are kept constant, the formulation of the precursor coatingcomposition will still play a significant role in the overall sprayingprocess and the resulting coating that is obtained.

In particular, the invention comprises a precursor coating compositioncomprising a liquid mixture of:

(a) a solids fraction containing at least one polymeric compound capableof forming a coating on a substrate; and

(b) a solvent fraction containing at least one coupling solvent in whichsaid at least one polymeric compound is at least partially soluble andwhich is at least partially miscible with water; and

(c) water, which is present in an amount of less than about 30% byweight based on the weight of the solvent fraction;

said liquid mixture having:

(i) a viscosity of less than about 6,000 centipoise and having less thanabout 650 grams of the solvent fraction per liter of mixture;

(ii) a solubility with at least one supercritical fluid, above thecritical temperature and pressure of the supercritical fluid, of greaterthan 5% by weight of supercritical fluid in said mixture;

(iii) a viscosity of less than about 300 centipoise when admixed with asufficient amount of the at least one supercritical fluid, above thecritical temperature and pressure of the supercritical fluid, so as torender the mixture suitable for spray application; and

(iv) a solubility with the supercritical fluid in the non-supercriticalstate, at 25° C. and one atmosphere absolute pressure of said fluid, ofless than about 0.8% by weight of fluid in said mixture.

As used herein, the "critical temperature" is defined as the temperatureabove which a gas cannot be liquefied by an increase in pressure. Also,as used herein, the "critical pressure" is defined as that pressurewhich is just sufficient to cause the appearance of two phases at thecritical temperature.

The invention is also directed to precursor coating compositions asdescribed above to which pigments, pigment extenders, metallic flakes,fillers, drying agents, anti-foaming agents, anti-skinning agents,wetting agents, ultraviolet absorbers, cross-linking agents, and othercoating additives are admixed with (a) and (b).

The present invention is also directed to processes for the liquid sprayapplication of the coatings discussed above to a substrate wherein theuse of environmentally undesirable volatile organic solvents andnon-aqueous diluents may even further be diminished than that realizedin the above noted related applications. Accordingly, the process of thepresent invention comprises:

(a) forming a liquid mixture in a closed system, said liquid mixturecomprising:

(i) a solids fraction containing at least one polymeric compound capableof forming a coating on a substrate;

(ii) a solvent fraction containing at least one coupling solvent inwhich said at least one polymeric compound is at least partially solubleand which is at least partially miscible with water;

(iii) water, which is present in an amount of less than about 30% byweight based on the weight of the solvent fraction; and

(iv) at least one supercritical fluid, in at least an amount which whenadded to (i), (ii), and (iii) is sufficient to render the viscosity ofsaid mixture to a point suitable for spray application; and

(b) spraying said liquid mixture onto a substrate to form a liquidcoating thereon.

As used herein, the terms "liquid spray", "liquid droplets" or "liquidcoating" is meant to define a spray, droplet, or coating containing aportion of the solids fraction, a portion of the solvent fraction, aportion of the water, in addition to any entrained supercritical fluidthat may still be present in such spray, droplet or coating.

In both the precursor coating composition as well as in the processusing such precursor composition to apply the same to a substrate, inaddition to the water, what is generally also desirable is the use of acoupling solvent. Such coupling solvent enables the presence of a singlephase in the water-containing composition such that the components ofthe composition, namely, the polymeric components, the water, and theorganic solvent all are at least partially miscible with one another. Ifdesired, all of the organic solvent in the composition may be a couplingsolvent. At the very least, the precursor composition contains polymericcomponent, water and such coupling solvent. The presence of an activesolvent which, as used herein, is an organic solvent in which saidpolymeric compound is at least partially soluble and which is also atleast partially miscible with the supercritical fluid, while desirable,is nevertheless optional. Such active solvent would be used inconjunction with a coupling solvent or, may actually be one and thesame.

As used herein, it is understood that the phrase "precursor coatingcomposition" is a composition which is primarily intended andparticularly suitable for admixture with at least one supercriticalfluid for subsequent liquid spraying onto a substrate to provide aliquid coating thereon which, when dried or cured, helps produce asubstantially uniform, continuous, substantially defect-free coating.However, if desired, this precursor coating composition may, of course,be utilized for an entirely different purpose although such use wouldnot constitute the preferred objective of the present invention. Thus,the precursor coating composition may be utilized, if desired, by simplyadding a suitable solvent to the composition (other than a supercriticalfluid) and then using such a diluted composition as a coating medium inany conventional manner. It is to be understood that the scope of thepresent invention is not narrowly limited to using the precursor coatingcomposition only with supercritical fluid and then spraying theresulting admixture. The precursor coating composition of the presentinvention is believed to be unique regardless of the manner in which itis eventually used.

Also as used herein, it is understood that the phrases "admixed coatingcomposition" or "admixed liquid mixture" are intended to mean asprayable mixture of the precursor coating composition and at least onesupercritical fluid.

It should be noted and stressed that the above noted instances in whichit would be desirable to utilize water as an additional viscosityreducing diluent are only exemplary. Other situations may arise, whenusing supercritical fluids as a viscosity reduction diluent, in whichthe use of water as yet an additional diluent would be applicable. Ofcourse, the scope of the present invention includes such additionalapplications as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram of supercritical carbon dioxide fluid spraycoating.

FIG. 2 is a graph illustrating the viscosity versus compositionrelationship for a 65% viscous polymer solution in methyl amyl ketone.

FIG. 3 is a schematic diagram of a spray apparatus that can be used withthe precursor coating composition of the present invention.

FIG. 4 is a schematic diagram of yet another spray apparatus that can beused with the precursor coating composition of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Because of its importance to the claimed invention, a brief discussionof relevant supercritical fluid phenomena is warranted.

The supercritical fluid phenomenon is well documented, see pagesF-62-F-64 of the CRC Handbook of Chemistry and Physics, 67th Edition,1986-1987, published by the CRC Press, Inc., Boca Raton, Fla. At highpressures above the critical point, the resulting supercritical fluid,or "dense gas", will attain densities approaching those of a liquid andwill assume some of the properties of a liquid. These properties aredependent upon the fluid composition, temperature, and pressure. As usedherein, the "critical point" is the transition point at which the liquidand gaseous states of a substance merge with each other and representthe combination of the critical temperature and critical pressure for agiven substance.

The compressibility of supercritical fluids is great just above thecritical temperature where small changes in pressure result in largechanges in the density of the supercritical fluid. The "liquid-like"behavior of a supercritical fluid at higher pressures results in greatlyenhanced solubilizing capabilities compared to those of the"subcritical" compound, with higher diffusion coefficients and anextended useful temperature range compared to liquids. Compounds of highmolecular weight can often be dissolved in the supercritical fluid atrelatively low temperatures. An interesting phenomenon associated withsupercritical fluids is the occurrence of a "threshold pressure" forsolubility of a high molecular weight solute. As the pressure isincreased, the solubility of the solute will often increase by manyorders of magnitude with only a small pressure increase. The solventcapabilities of the supercritical fluid, however, are not essential tothe broad aspects of the invention.

Near-supercritical liquids also demonstrate solubility characteristicsand other pertinent properties similar to those of supercritical fluids.The solute may be a liquid at the supercritical temperatures, eventhough it is a solid at lower temperatures. In addition, it has beendemonstrated that fluid "modifiers" can often alter supercritical fluidproperties significantly, even in relatively low concentrations, greatlyincreasing solubility for some solutes. These variations are consideredto be within the concept of a supercritical fluid as used in the contextof this invention. Therefore, as used herein, the phrase "supercriticalfluid" denotes a compound above, at, or slightly below the criticaltemperature and pressure (the critical point) of that compound.

Examples of compounds which are known to have utility as supercriticalfluids are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        EXAMPLES OF SUPERCRITICAL SOLVENTS                                                      Boiling   Critical   Critical                                                                             Critical                                          Point     Temperature                                                                              Pressure                                                                             Density                                 Compound  (°C.)                                                                            (°C.)                                                                             (atm)  (g/cm.sup.3)                            ______________________________________                                        CO.sub.2  -78.5     31.3       72.9   0.448                                   NH.sub.3  -33.35    132.4      112.5  0.235                                   H.sub.2 O 100.00    374.15     218.3  0.315                                   N.sub.2 O -88.56    36.5       71.7   0.45                                    Xenon     -108.3    16.6       57.6   0.118                                   Krypton   -153.2    -63.8      54.3   0.091                                   Methane   -164.00   -82.1      45.8   0.2                                     Ethane    -88.63    32.28      48.1   0.203                                   Ethylene  -103.7    9.21       49.7   0.218                                   Propane   -42.1     96.67      41.9   0.217                                   Pentane   36.1      196.6      33.3   0.232                                   Methanol  64.7      240.5      78.9   0.272                                   Ethanol   78.5      243.0      63.0   0.276                                   Isopropanol                                                                             82.5      235.3      47.0   0.273                                   Isobutanol                                                                              108.0     275.0      42.4   0.272                                   Chlorotri-                                                                              -31.2     28.0       38.7   0.579                                   fluoromethane                                                                 Monofluoro-                                                                             -78.4     44.6       58.0   0.3                                     methane                                                                       Cyclohexanol                                                                            155.65    356.0      38.0   0.273                                   ______________________________________                                    

Due to the low cost, environmental acceptability, non-flammability, andlow critical temperature of carbon dioxide, supercritical carbon dioxidefluid is preferably used with the precursor coating compositions of thepresent invention. For many of the same reasons, nitrous oxide (N₂ O) isa desirable supercritical fluid for admixture with the precursor coatingcompositions of the present invention. However, any of theaforementioned supercritical fluids and mixtures thereof are to beconsidered as being applicable for use with the precursor coatingcompositions.

The solvency of supercritical carbon dioxide is substantially similar tothat of a lower aliphatic hydrocarbon and, as a result, one can considersupercritical carbon dioxide as a replacement for the hydrocarbonsolvent of a conventional coating formulation. In addition to theenvironmental benefit of replacing hydrocarbon solvents withsupercritical carbon dioxide, there is a safety benefit also, becausecarbon dioxide is non-flammable.

To better understand the phenomenon that is occurring when asupercritical fluid, such as supercritical carbon dioxide, is added to aprecursor coating composition and the problems that may be encountered,reference is made to the phase diagram in FIG. 1 wherein thesupercritical fluid is supercritical carbon dioxide fluid. In FIG. 1,the vertices of the triangular diagram represent the pure components ofan admixed coating composition which for the purpose of this discussioncontains no water. Vertex A is an organic solvent, vertex B is carbondioxide, and vertex C represents a polymeric material. The curved lineBFC represents the phase boundary between one phase and two phases. Thepoint D represents a possible composition of a coating composition inwhich supercritical carbon dioxide has not been added. The point Erepresents a possible composition of an admixed coating composition,after admixture with supercritical carbon dioxide. Generally, theaddition of supercritical carbon dioxide reduces the viscosity of theviscous precursor coating composition to a range where it can be readilyatomized through a liquid spray apparatus.

Thus, after atomization, a majority of the carbon dioxide vaporizes,leaving substantially the composition of the original viscous coatingcomposition. Upon contacting the substrate, the remaining liquid mixtureof the polymer and solvent(s) component(s) will flow, i.e., coalesce, toproduce a uniform, smooth film on the substrate. The film formingpathway is illustrated in FIG. 1 by the line segments EE'D (atomizationand decompression) and DC (coalescence and film formation).

However, the addition of supercritical carbon dioxide to a viscouscoating composition does not always result in a viscosity reductionwhich is sufficient to allow for spraying of the composition. Suchviscosity reduction is limited to the amount of supercritical fluid,such as supercritical carbon dioxide, that can be admixed with thecoating composition. If not enough supercritical fluid can be added,then the viscosity of the composition is not lowered enough to make itsuitable for spray application. This limitation as to the amount ofsupercritical fluid that can be added to the composition is generally afunction of the miscibility of the supercritical fluid with the coatingcomposition and can best be visualized by again referring to FIG. 1.

As can be seen from the phase diagram, particularly as shown by arrow10, as more and more supercritical carbon dioxide is added to thecoating composition in an attempt to reduce its viscosity sufficiently,the composition of the admixed liquid coating mixture approaches thetwo-phase boundary represented by line BFC. If enough supercriticalcarbon dioxide is added, the two-phase region is reached and thecomposition correspondingly breaks down into two phases. Sometimes, itmay be desirable to admix an amount of supercritical fluid (in thiscase, supercritical carbon dioxide) which is even beyond the two phaseboundary. Generally, however, it is not preferable to go much beyondthis two phase boundary for optimum spraying performance and/or coatingformation. This two-phase region may be reached, however, prior toachieving the necessary viscosity reduction. Any additionalsupercritical carbon dioxide added to the system beyond this point willgenerally not aid in any further viscosity reduction.

Viscosity reduction brought about by adding supercritical carbon dioxidefluid to viscous coatings composition is illustrated in FIG. 2. There,the viscous coating composition of 65 percent polymer solution in methylamyl ketone, which corresponds to point D in FIG. 1 has a viscosity ofabout 300 centipoise and the solution is unsprayable. Addingsupercritical carbon dioxide fluid to the coating composition reducesthe viscosity such that a liquid mixture that contains 28 percentsupercritical carbon dioxide fluid, which corresponds to point E in FIG.1, is formed. This liquid mixture now has a viscosity of less than 30centipoise and readily forms a liquid spray by passing it through anorifice in an airless spray gun. The pressure utilized in FIG. 2 is 1250psi at a temperature of 50° C. The polymer is Acryloid™ AT-400, aproduct of Rohm and Haas, which contains 75 percent of non-volatileacrylic polymer dissolved in 25 percent methyl amyl ketone.

In accordance with the present invention, however, water may be added toa highly viscous coating composition as a further viscosity reducingdiluent, which water-containing composition may then be admixed withsupercritical fluids. Regardless of how the coating composition has cometo have an unusually high viscosity, whether it be due to the nature ofthe polymer itself (e.g., its molecular weight) or the nature and/oramount of the solvent present in the composition, the addition of thewater will generally aid in reducing the viscosity to the extent suchthat when admixed with the supercritical fluids, the resulting admixedcomposition containing such water and supercritical fluids will besuitable for spraying.

Generally, the amount of water that is present in the composition is upto about 30 percent by weight based on the weight of the total solventfraction contained in the composition. Preferably, the amount of wateris less than about 20 percent by weight on that same basis.

Higher quantities of water than those noted above may be not bedesirable for a number of reasons. For one, too much water may alsoresult in a phase separation, i.e., the composition breaks down into awater phase and an organic solvent phase. Such a phase separation, as inthe case of a phase separation caused by an excessive amount ofsupercritical fluid, may result in poor spraying performance and/or poorcoating formation.

Thus, without wishing to be bound by theory, once such a phaseseparation takes place, the separate water phase may attractsupercritical fluid and organic solvent leaving less organic solventpresent in the separate organic solvent phase. This may result in aviscosity imbalance between the two phases which may very well hinder orprevent the spray application of the coating composition.

So too, it is also believed, again without wishing to be bound bytheory, that the concentration of the organic solvent in the water phasemay be such that even if the composition were sprayed, there may be anexcessive evaporation of such solvent resulting in an insufficent amountof solvent on the substrate to allow for proper coalescence of theatomized particles resulting in a poor coating.

For similar reasons, in the case where water is added to a coatingcomposition so as to replace a portion of the organic solvent presenttherein and thereby further reduce the overall organic solvent content,there should still be enough solvent present which will facilitate theproper flow-out and coalescence of the sprayed particles on thesubstrate to form a desirable coating thereon.

Accordingly, it is generally desirable to add as much water as may benecessary so as to provide a precursor coating composition whoseviscosity may be reduced to a point suitable for spray application bysubsequent admixture with supercritical fluids.

Although the above discussion has focused upon those cases in which theamount of supercritical fluid added to a viscous composition isineffective to reduce the viscosity to the extent necessary in order tospray such composition and adding water to these compositions inaccordance with the present invention so as to be able to spray them, itis understood that the addition of water to a coating composition is notlimited to only those conditions. Indeed, water may be added to acoating composition for subsequent admixture with supercritical fluidseven when the viscosity of the original composition is such that theaddition of supercritical fluids is indeed capable of reducing theviscosity to a point suitable for spraying.

For example, in the case where it is desirable to simply reduce theamount of volatile organic solvent present in a first coatingcomposition where such first coating composition could be sprayed whenadmixed with supercritical fluids, it is clear that when water is usedto replace some of the volatile solvent contained therein, there is noconcern here as to phase separation caused by an excessive amount ofsupercritical fluid.

As briefly discussed earlier, even after water is added to a coatingcomposition, it has been found by virtue of the present invention thatthe amount of supercritical fluid that is capable of being admixed withthe now water-containing composition, in absolute terms, remainssubstantially the same. In other words, if, for example, 100 grams ofsupercritical carbon dioxide were able to be admixed with a particularcomposition, after adding water to this composition, approximately 100grams of supercritical carbon dioxide can still be admixed with the nowwater-containing composition.

That is not to say, however, that it is necessary to add the same amountof carbon dioxide in the new water-containing composition as was capableof being added to the original composition. Although it is beneficial tomaximize the amount of supercritical fluids that are utilized asdiluents for viscosity reduction to thereby keep the solvent fraction,particularly the organic solvents and/or non-aqueous diluents to aminimum, there is no criticality as to the amount of supercriticalfluids that are used other than using an amount which will produce asprayable composition.

However, the capability of the water-containing composition to beadmixed with the same amount of supercritical fluids that was able to beused in the "dry" composition is significant inasmuch as the concomitantdiluent effect that is obtained by the supercritical fluid can beexpected to also remain substantially constant.

In addition to the water that is present in the precursor coatingcompositions of the present invention, which water is used as a furtherviscosity reducing diluent, it is generally also desirable to have acoupling solvent present in the precursor coating composition as well.Of course, if a coating composition to which the water is added alreadycontains an organic solvent which may be characterized as a couplingsolvent, no further addition of such a coupling solvent need be made.Similarly, if a precursor composition is being initially prepared withwater, at least one of the organic solvents used for such compositionshould desirably be a coupling solvent.

A coupling solvent is a solvent in which the polymeric compounds used inthe solids fraction is at least partially soluble. Most importantly,however, such a coupling solvent is also at least partially misciblewith water. Thus, the coupling solvent enables the miscibility of thesolids fraction, the solvent fraction and the water to the extent that asingle phase is desirably maintained such that the composition mayoptimally be sprayed and a good coating formed.

Coupling solvents are well known to those skilled in the art and anyconventional coupling solvents which are able to meet the aforementionedcharacteristics, namely, those in which the polymeric components of thesolid fraction is at least partially soluble and in which water is atleast partially miscible are all suitable for being used in the presentinvention.

Applicable coupling solvents which may be used in the present inventioninclude, but are not limited to, ethylene glycol ethers, propyleneglycol ethers, chemical and physical combinations thereof; lactams;cyclic ureas; and the like.

Specific coupling solvents (which are listed in order of mosteffectiveness to least effectiveness) include butoxy ethanol, propoxyethanol, hexoxy ethanol, isopropoxy 2-propanol, butoxy 2-propanol,propoxy 2-propanol, tertiary butoxy 2-propanol, ethoxy ethanol, butoxyethoxy ethanol, propoxy ethoxy ethanol, hexoxy ethoxy ethanol, methoxyethanol, methoxy 2-propanol, and ethoxy ethoxy ethanol. Also includedare lactams such as n-methyl-2-pyrrolidone, and cyclic ureas such asdimethyl ethylene urea.

In addition to a coupling solvent, it may also be desirable to add (orhave present) an active solvent as well. An active solvent, as usedherein, is meant to include those solvents which have particularly goodsolubility for the polymeric compounds that are used in the compositionin addition to having at least partial miscibility with supercriticalfluids.

Suitable active solvents which may be utilized in the precursor coatingcompositions of the present invention include ketones such as acetone,methyl ethyl ketone, methyl isobutyl ketone, mesityl oxide, methyl amylketone, cyclohexanone and other aliphatic ketones; esters such as methylacetate, ethyl acetate, alkyl carboxylic esters; ethers such as methylt-butyl ether, dibutyl ether, methyl phenyl ether and other aliphatic oralkyl aromatic ethers; glycol ethers such as ethoxy ethanol, butoxyethanol, ethoxy 2-propanol, propoxy ethanol, butoxy 2-propanol and otherglycol ethers; glycol ether esters such as butoxyethoxy acetate, ethyl3-ethoxy propionate and other glycol ether esters; alcohols suchmethanol, ethanol, propanol, iso-propanol, butanol, iso-butanol, amylalcohol and other aliphatic alcohols; aromatic hydrocarbons such astoluene, xylene, and other aromatics or mixtures of aromatic solvents;aliphatic hydrocarbons such as VM&P naphtha and mineral spirits, andother aliphatics or mixtures of aliphatics; nitro alkanes such as2-nitropropane. A review of the structural relationships important tothe choice of solvent or solvent blend is given by Dileep et al., Ind.Eng. Che. (Product Research and Development) 24, 162, 1985 and Francis,A. W., J. Phys. Chem. 58, 1099, 1954.

Of course, there are solvents which can function both as couplingsolvents as well as active solvents and the one solvent may be used toaccomplish both purposes. Such solvents include, for example, butoxyethanol, propoxy ethanol, and propoxy 2-proponal. Glycol ethers areparticularly preferred.

When using both a coupling solvent as well as an active solvent in theprecursor coating compositions of the present invention, the ratio ofcoupling solvent to active solvent by weight is generally in the rangeof from about 1:1 to 4:1, preferably about 2:1 to 3.5:1, and mostpreferably about 2.5:1 to 3:1. Generally, the ratio of coupling solventto active solvent will be dependent, among other things, on thehydrophobicity of the active solvent.

The presence of too much coupling solvent may interfere with thedissolution of the polymer while the presence of too much active solventmay interfere with proper miscibility of the water.

While the polymeric compounds that are suitable for use in the presentinvention as coating materials generally include any of the polymerswhich are well known to those skilled in the coatings art, there arepreferred polymers which are particularly desirable due to theirpossessing specific characteristics which make them generally moresuitable for (1) admixture with supercritical fluids followed by (2)spraying such admixture onto a substrate so as to help obtain a defectfree coating.

Generally, the polymers which may be used in the present invention mustbe able to withstand the temperatures and/or pressures which areinvolved when they are ultimately admixed with the at least onesupercritical fluid. Such applicable polymers include thermoplastic orthermosetting materials or may be crosslinkable film forming systems.

In particular, the polymeric components include vinyl, acrylic,styrenic, and interpolymers of the base vinyl, acrylic, and styrenicmonomers; polyesters, oil-free alkyds, alkyds, and the like;polyurethanes, oil-modified polyurethanes and thermoplastic urethanessystems; epoxy systems; phenolic systems; cellulosic esters such asacetate butyrate, acetate propionate, and nitrocellulose; amino resinssuch as urea formaldehyde, melamine formaldehyde, and other aminoplastpolymers and resins materials; natural gums and resins; and enamels,varnishes, and lacquers. Also included are mixtures of the above coatingmaterials commonly used and known to those skilled in the art that areformulated to achieve performance and cost balances required ofcommercial coatings.

One characteristic which is possessed by particularly preferred polymersthat are used in the present invention are those having a low elasticcomponent of viscosity. A discussion of the components of viscosity canbe found in, for example, "Rheological Measurement for Quality Control"by S. B. Driscoll, Rubber World (December, 1980), pages 31-34, thecontents of which are incorporated by reference. Thus, where a number ofpolymers may provide a precursor coating composition having essentiallythe same viscosity, the most preferred polymer would be the one havingthe least elastic component of viscosity. Such polymers having a lowelastic component of viscosity are generally those having a structuraland molecular weight distribution which, in solution, minimizes chainentanglement. Particularly, the high molecular weight distribution ofthe polymeric compound should be minimized. A useful and conventionalmeasure for the high molecular weight fraction of the molecular weightdistribution is the ratio of the weight average (Mw) molecular weight ofthe polymeric compound to the number average (Mn) molecular weight ofthat polymeric compound, i.e., Mw/Mn. Reference is made to, for example,"Introduction to Polymers and Resins", Federation Series On CoatingsTechnology (1986), pages 26-31, which discusses molecular weightdetermination of polymers. the contents of which are incorporated hereinby reference. Generally, for a given number average molecular weight,Mn, the higher the ratio of Mw/Mn, the greater the high molecular weightfraction that is present in the polymer, and the greater the elasticcomponent of viscosity possessed by such polymer.

Preferably, the predominant polymeric compound used in the precursorcomposition of the present invention has a Mw/Mn ratio of less thanabout 4, and preferably less than about 3, and most preferably less thanabout 2.

The higher the elastic component of viscosity possessed by the polymer,the more elastic a polymer is, the more difficult it is to atomize anadmixed precursor coating composition containing supercritical fluidmade from such a polymer. Generally, as an admixed coating compositionis released through the spray orifice, shearing forces act upon thematerial causing it to tear itself apart into fine atomized droplets.When, however, the polymer has a high elastic component of viscosity,such tearing apart is hindered and the composition does not atomize aswell.

Particularly desirable polymers having a relatively low elasticcomponent of viscosity include those set forth in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        Polymer M.sub.n       M.sub.w       M.sub.w /M.sub.n                          ______________________________________                                        Alkyd   25,000-50,000  50,000-200,000                                                                             2-4                                       resins                                                                        Epoxy     350-4,000     350-7,000   1.0-2.5                                   resins                                                                        Acrylic,                                                                               25,000-350,000                                                                              40,000-600,000                                                                             1.5-3                                     thermo-                                                                       plastic,                                                                      solution                                                                      polymer                                                                       Acrylic,                                                                                500,000-2,000,000                                                                           650,000-2,500,000                                                                         1.1-1.8                                   thermo-                                                                       plastic,                                                                      emulsion                                                                      polymer                                                                       Acrylic 1,000-2,000   1,200-2,200   1.1-1.5                                   thermo-                                                                       setting,                                                                      oligomer                                                                      Poly-   2,000-5,000   2,100-5,200   1.05-1.1                                  butadiene,                                                                    anionic                                                                       poly-                                                                         merized                                                                       ______________________________________                                    

In addition to the polymeric compound that is contained in the solidsfraction, additives which are typically utilized in coatings may also beused. For example, pigments, pigment extenders, metallic flakes,fillers, drying agents, anti-foaming agents, and anti-skinning agents,wetting agents, ultraviolet absorbers, cross-linking agents, andmixtures thereof, may all be utilized in the precursor composition ofthe present invention.

In connection with the use of the various additives noted above, it isparticularly desirable for pigments to be present in the precursorcomposition inasmuch as it has been found to aid in the diffusion of thesupercritical fluid from the sprayed composition resulting in improvedatomization.

The solvent fraction of the precursor composition of the presentinvention includes at least one coupling solvent in which the at leastone polymer compound is at least partially soluble and which is at leastpartially miscible with water, as discussed earlier. Optionally, anactive solvent, as noted above, may also be employed. While the solventsthat are suitable for use in the present invention generally include (aslong as there is a coupling type solvent present) any solvent ormixtures of solvent which is capable of dissolving, dispersing orsuspending the solids fraction when admixed with the supercriticalfluid, as with the solids fraction discussed above, there are preferredsolvents and solvent mixtures which are particularly desirable. Suchpreferred solvents, either coupling and/or active, possess certaindesirable characteristics which make them generally more suitable foradmixture with a supercritical fluid followed by spraying such admixtureonto a substrate material.

As is quite apparent, the selection of a particular solvent or solventmixture will generally be dependent upon the particular polymericcompounds being used.

In general, solvents suitable for the present invention should have thedesired solvency characteristics as aforementioned and also the properbalance of evaporation rates so as to ensure coating formation. In otherwords, the solvent fraction should have a proper blend of fast and slowsolvents.

More specifically, solvents having fast evaporation rates are needed tohelp solubilize the solids fraction, as well as help reduce viscosity,and to substantially leave the admixed coating composition once it hasbeen sprayed and before the atomized droplets contact the substrate.Correspondingly, solvents having slow evaporation rates are also neededto help solubilize the solids fraction, but these solvents are primarilyneeded to be present on the substrate after the atomized droplets havebeen applied so as to provide a suitably low viscosity to produce enoughflow-out to obtain a uniform and continuous coating.

However, too much of the fast solvent will produce a dry coating nothaving enough flow-out. Conversely, too much of the slow solvent willproduce a coating having sagging and running defects and will notreadily be dried thereby hindering the early handling of such a coatedsubstrate.

Based on a relative evaporation rate (RER) to a butyl acetate standardequal to 100 using ASTM Method D3599 at 25° C. and one atmospherepressure, the solvent fraction desirably has the following compositionof fast and slow solvents as represented by corresponding RER values:

    ______________________________________                                        Wt. % of Total Solvent Fraction                                                                    RER                                                      ______________________________________                                        30-100%              <50                                                      0-70%                 50-100                                                  0-40%                101-250                                                  <10%                 >250                                                     ______________________________________                                    

More preferably, the solvent fraction has the following composition:

    ______________________________________                                        Wt. % of Total Solvent Fraction                                                                    RER                                                      ______________________________________                                        40-100%              <50                                                      0-60%                 50-100                                                  0-30%                101-250                                                  <5%                  >250                                                     ______________________________________                                    

Another characteristic of the solvent fraction which desirably isoptimized so as to make it particularly suitable for admixture with asupercritical fluid with subsequent spraying thereof is the surfacetension of the solvent fraction. Specifically, the less surface tensionthat a solvent has, the more desirable it is.

Accordingly, solvents having low surface tension provide goodatomization of the admixed precursor coating composition providing afine aerosol. Such a fine aerosol desirably facilitates the escape ofthe supercritical fluid from the sprayed admixed coating compositionprior to its contacting the substrate.

Moreover, solvents having low surface tension also facilitate theformation of good coatings on the substrate by aiding in the diffusionof any residual supercritical fluid that may still be entrapped withinthe applied coating. So too, low surface tension solvents also help tosuppress the formation of any bubbles caused by such residualsupercritical fluid as it escapes. Still further, solvents having lowsurface tension desirably provide fast wetting and spreadingcharacteristics which also aid in the formation of a defect-free,uniform coating on the substrate.

Preferably, the solvent fraction has a surface tension at 25° C. whichis desirably less than 35 dynes/centimeter. More preferably, the surfacetension is less than 30 dynes/centimeter, for example, 23-28dynes/centimeter.

It is understood that it is not necessary that the solvent or solventmixture possess such surface tension characteristics per se. While suchsolvents do certainly exist, as exemplified by such solvents as toluene,VM&P naphtha, butyl acetate, pentyl propionate, glycol ethers, methylPROPASOL® acetate (manufactured by Union Carbide Corp., Danbury, Conn.),UCAR® Ester EEP (manufactured by Union Carbide Corp., Danbury, Conn.),and the like, there are conventional additives which may be added to theprecursor composition which contains the solvent and solids fractions soas to suppress the surface tension of the composition as a whole. Suchadditives include surface active materials, commonly known assurfactants, which are well known to those skilled in the art and whichare applicable to be used in both the solvent fraction of the presentinvention as well as in the precursor coating composition as a whole.

Still further in connection with the solvent fraction, as the admixedcoating composition containing the mixture of polymer, solvent, waterand supercritical fluid is sprayed, the vaporization of the fast solventcontributes to the overall cooling of the sprayed composition. Inasmuchas the solubility of most supercritical fluids, particularly carbondioxide, is higher at cooler temperatures, such cooling manifestlyhinders the diffusion of the supercritical fluid from the sprayedcomposition. It is desirable to have essentially all of thesupercritical fluid escape from the admixed coating composition, once ithas been sprayed, such that essentially none of the supercritical fluidis left once the atomized liquid droplets contact the substrate.

In order to help minimize the cooling effect caused by evaporation ofthe fast solvent, it is desirable that the solvent fraction have anoverall low heat of vaporization. Preferably, the solvent fraction hasan overall heat of vaporization of less than 110 calories per gram ofsolvent fraction and more preferably, less than about 100 calories pergram of solvent fraction. Desirably, the overall heat of vaporization ofthe solvent fraction is in the range of from about 65 to 95calories/gram.

Keeping in mind the underlying primary objective of the presentinvention, namely, to minimize the unnecessary release of solvent vaporsinto the atmosphere during the spray application of the admixed coatingcompositions, it is clear that the amount of solvent used in theprecursor coating compositions should be less than that required toproduce a mixture of polymeric compounds and solvent having a viscositywhich would permit its application by liquid spray techniques. In otherwords, the inclusion of the solvent fraction should be minimized suchthat the diluent effect due to the presence of the supercritical fluiddiluent is fully utilized.

However, reducing the amount of solvent in the coating composition isbeneficial not only for the purpose of minimizing environmentalpollution, but as recognized by the present invention, such reduction inthe amount of solvent is also desirable to help facilitate the sprayingof the coating composition once it has been admixed with supercriticalfluid as well as improving the appearance of the coating that isultimately formed on the substrate.

More specifically, as the amount of solvent fraction present in theprecursor coating composition is increased and after such compositionhas been admixed with supercritical fluid and sprayed, the rate ofdiffusivity of such supercritical fluid through the sprayed compositiongenerally decreases. This typically results in an increase of residualsupercritical fluid retained in the coating after it has been applied tothe substrate which may result in the formation of coating defects. Inorder to compensate for such decreased diffusivity, the temperature ofthe sprayed composition is generally raised so as to lower thesolubility of the supercritical fluid. However, such an increase inspray temperature may have an effect on the particle size and theatomization quality of the sprayed composition.

Still further, as the solvent fraction in the precursor coatingcomposition is increased, there is a corresponding loss in thedistinctness of image gloss (DOI) in the resulting coating. This isbelieved to be caused by a concomitant increase in the amount ofresidual supercritical fluid that is retained in the applied coatingwhich gradually fizzles out of the coating.

Furthermore, an excessive solvent fraction in the precursor coatingcomposition diminishes the effectiveness of the supercritical fluid,because atomization is intensified such that deposition of the liquidspray onto the substrate becomes poorer than when the solvent fractionis not excessive. That is, the transfer of liquid coating to thesubstrate becomes less efficient. Therefore, more spraying must be doneto build up coating thickness, with the result that more solvent isreleased to the environment and more precursor coating composition isused, which increases cost. In addition, because more spray passes mustbe used to build the desired coating thickness, a higher proportion ofslow solvent is lost from the coating during the application process, sothat less slow solvent is available to aid reflow of the coating to givea smooth surface once the desired coating thickness has been achieved.Without wishing to be bound by theory, it is believed that viscosityreduction caused by the excessive solvent fraction combines withviscosity reduction caused by the supercritical fluid to give dropletsthat have insufficient mass to be deposited efficiently onto thesubstrate from the spray. Instead, the droplets follow the gas flow intothe environment. Using less solvent allows full utilization of theviscosity reduction of the supercritical fluid and gives droplet sizesthat deposit efficiently onto the substrate.

Moreover, an excessive solvent fraction produces a greater wet coatingthickness for a desired dry coating thickness and a lower coatingviscosity on the substrate, which increases the tendency for the coatingto sag or run during application, drying, and curing. In addition, asthe excessive solvent evaporates from the coating, the coating shrinksto a greater degree during the drying process and flow currents can beinduced inside the coating which disrupt the leveling to give a smoothsurface. Drying time is also increased.

Consequently, in accordance with the present invention, the amount ofsolvent fraction that is present in the liquid mixture comprised of asolids fraction, a solvent fraction and water is no greater than about650 grams of solvent per liter of liquid mixture. More preferably, theamount of solvent fraction contained in the liquid mixture is less thanabout 450 grams of solvent per liter of liquid mixture. Most preferably,the amount of solvent fraction is in the range of from about 200 toabout 400 grams of solvent per liter of mixture.

The liquid precursor coating composition of the present inventioncomprising the solids fraction, the solvent fraction and water shouldhave a viscosity of less than about 6,000 centipoise, and preferablyless than about 3,000 centipoise, and most preferably in the range offrom about 500 to about 2,000 centipoise. Obviously, a major factorcontributing to the viscosity of the liquid mixture is the amount ofsolvent fraction and water contained therein, which variable wasdiscussed above. Hence, it is apparent that the amount of solventfraction present in the precursor coating composition should beconsidered hand-in-hand with the desired viscosity that is to beobtained.

The viscosity of the precursor coating composition should be low enoughsuch that there is enough solvent and water present to provide propercoalescence upon the substrate once the composition is sprayed whilestill being high enough to allow for a reduction in solvent usage so asto maximize the utilization of the supercritical fluid diluent and toconcomitantly facilitate good atomization and coating formation.

The viscosity of the precursor coating composition should also be suchthat when supercritical fluid is added, it is possible to add enough ofthe supercritical fluid, without entering the two phase region, suchthat the viscosity is lowered to less than about 300 centipoise, abovethe critical temperature and pressure of the supercritical fluid, so asto render the mixture suitable for spray application. More preferably,the admixed liquid coating composition has a viscosity which is lessthan about 150 centipoise and preferably has a viscosity in the range offrom about 5 to 150 centipoise. Most preferably, the viscosity of theadmixture of solids fraction, solvent fraction, water and supercriticalfluid is in the range of from about 10 to about 50 centipoise.

Yet another factor which the precursor coating composition must addressis the solubility of the supercritical fluid both at supercriticalconditions and at the conditions of the substrate, i.e., after thecomposition has been sprayed.

The solubility requirements for these two sets of conditions are totallyantithetical to one another. Thus, when admixing the supercritical fluidwith the liquid precursor composition, it is desirable to have acomposition which has a high solubility for the supercritical fluid atthe supercritical conditions. In contrast, once the composition has beensprayed through the orifice, it is desirable that the solubility for thesupercritical fluid at the conditions present in the environment of thesubstrate be as low as possible.

More particularly, in accordance with the present invention, the liquidprecursor coating composition should have a solubility for thesupercritical fluid, above the critical temperature and pressure of thesupercritical fluid, of at least 5% by weight of the supercritical fluidin the liquid mixture. Preferably, the solubility should be at least 15%by weight of the supercritical fluid in the mixture and, more preferablyabout 20 to 50% or greater by weight of supercritical fluid in themixture. Most preferably, it is in the range of from about 25% to about35% by weight.

If the precursor coating composition has a solubility which is less thanthat noted above, there simply would not be enough of a diluent effectprovided by the supercritical fluid. This would result in aninsufficient viscosity reduction such that the composition could notproperly be sprayed.

Once the composition is admixed with supercritical fluid and sprayed, itis desirable to have the supercritical fluid diffuse through the sprayedcomposition as quickly as possible such that there is very littleresidual supercritical fluid left in the coating once it has come intocontact with the substrate. Accordingly, the fluid, which of course isno longer supercritical, should have a solubility in the liquidprecursor coating composition of less than about 0.8% by weight of thefluid in the non-supercritical state in the precursor coatingcomposition. Preferably, the solubility of the fluid is less than about0.6% by weight in the composition. Most preferably, the fluid should besoluble in the precursor coating composition in an amount of less thanabout 0.4% by weight. As used herein, it is to be understood that thesolubility of the fluid in the non-supercritical state is measured at25° C. and in one atmosphere absolute pressure of the fluid.

A still further characteristic which the precursor coating compositionshould desirably possess is a high diffusivity for passing thesupercritical fluid out of the composition once it has been sprayed withsuch supercritical fluid into the environment of the substrate. Suchhigh diffusivities are desirable to help the supercritical fluid quicklyescape from the sprayed liquid mixture. This helps prevent the presenceof any residual supercritical fluid in the liquid coating applied to thesubstrate and thereby helps ensure the formation of a uniform,defect-free coating.

Accordingly, the precursor coating composition should desirably possessan apparent gas diffusion rate (based on a sprayed particle averagevelocity of about 2 to about 8 m/sec and a spraying distance of about 30cm at 25° C. and one atmosphere pressure) from the time it has beensprayed with supercritical fluid to the time it impacts on the substrateof at least about 7 to about 26 grams of supercritical fluid per gram ofpresursor coating composition per second.

The precursor coating composition, after having been admixed withsupercritical fluid, is sprayed onto a substrate to form a liquidcoating thereon containing solids fraction, a portion of the solventfraction, a portion of the water and any residual supercritical fluidthat may be left by passing the admixed liquid mixture under pressurethrough an orifice into the environment of the substrate to form aliquid spray.

An orifice is a hole or an opening in a wall or housing, such as in aspray tip of a spray nozzle on an electrostatic spray gun, through whichthe admixed liquid coating composition flows in going from a region ofhigher pressure, such as inside the spray gun, into a region of lowerpressure, such as the air environment, outside of the spray gun andaround the substrate. An orifice may also be a hole or an opening in thewall of a pressurized vessel, such as a tank or cylinder. An orifice mayalso be the open end of a tube or pipe or conduit through which themixture is discharged. The open end of the tube or pipe or conduit maybe constricted or partially blocked to reduce the open area.

Spray orifices, spray tips, spray nozzles, and spray guns used forconventional electrostatic airless and air-assisted airless spraying ofcoating formulations such as paints, lacquers, enamels, and varnishes,are suitable for spraying the precursor coating compositions of thepresent invention when admixed with supercritical fluids. Spray guns,nozzles, and tips are preferred that do not have excessive flow volumebetween the orifice and the valve that turns the spray on and off. Thespray guns, nozzles, and tips must be built to contain the spraypressure used.

There are a broad variety of spray devices that one may use in sprayingthe precursor coating composition of the present invention after it hasbeen admixed with supercritical fluid. Essentially, any spray gun may beused, from conventional airless and air-assisted airless spray devicesto electrostatic spray devices. The choice of spray device is dependentupon the kind of application that is contemplated.

The material of construction of the orifice through which the admixedcoating composition is sprayed must possess the necessary mechanicalstrength for the high spray pressure used, have sufficient abrasionresistance to resist wear from fluid flow, and be inert to chemicalswith which it comes into contact. Any of the materials used in theconstruction of airless spray tips, such as boron carbide, titaniumcarbide, ceramic, stainless steel or brass, is suitable, with tungstencarbide generally being preferred.

The orifice sizes suitable for spraying the admixed liquid mixtureshould generally range from about 0.004-inch to about 0.072-inchdiameter. Because the orifices are generally not circular, the diametersreferred to are equivalent to a circular diameter. The proper selectionis determined by the orifice size that will supply the desired amount ofliquid coating and accomplish proper atomization for the coating.Generally, smaller orifices are desired at lower viscosity and largerorifices are desired at higher viscosity. Smaller orifices give fineratomization but lower output. Larger orifices give higher output butpoorer atomization. Finer atomization is preferred in the practice ofthe present invention. Therefore, small orifice sizes from about0.004-inch to about 0.025-inch diameter are preferred. Orifice sizesfrom about 0.007-inch to about 0.015-inch diameter are most preferred.

The designs of the spray tip that contains the spray orifice and of thespray nozzle that contains the spray tip are not critical. The spraytips and spray nozzles should have no protuberances near the orificethat would interfere with the spray.

The shape of the spray is also not critical to being able to spray theadmixed coating composition. The spray may be in the shape of a conethat is circular or elliptical in cross section or the spray may be inthe shape of a flat fan, but the spray is not limited to these shapes.Sprays that are flat fans or cones that are elliptical in cross sectionare preferred.

The distance from the orifice to the substrate is generally at adistance of from about 4 inches to about 24 inches. A distance of 6inches to 18 inches is preferred. A distance of 8 inches to 14 inches ismost preferred.

Devices and flow designs that promote turbulent or agitated flow in theliquid mixture prior to passing the liquid mixture under pressurethrough the orifice may also be used. Such techniques include but arenot limited to the use of pre-orifices, diffusers, turbulence plates,restrictors, flow splitters/combiners, flow impingers, screens, baffles,vanes, and other inserts, devices, and flow networks that are used inelectrostatic airless spray and air-assisted airless spray.

Filtering the liquid mixture prior to flow through the orifice isdesirable in order to remove particulates that might plug the orifice.This can be done using conventional high-pressure paint filters. Afilter may also be inserted at or in the gun and a tip screen may beinserted at the spray tip to prevent orifice plugging. The size of theflow passages in the filter should be smaller than the size of theorifice, preferably significantly smaller.

Electrostatic forces may be used to increase the proportion of precursorcoating composition that is deposited onto a substrate from the spray.This is commonly referred to as increasing the transfer efficiency. Thisis done by using a high electrical voltage relative to the substrate toimpart an electrical charge to the spray. This creates an electricalforce of attraction between the spray droplets and the substrate, whichcauses droplets that would otherwise miss the substrate to be depositedonto it. When the electrical force causes droplets to be deposited onthe edges and backside of the substrate, this effect is commonlyreferred to as wrap around.

Preferably the substrate is grounded, but it may also be charged to theopposite sign as the spray. The substrate may be charged to the samesign as the spray, but at a lower voltage with respect to ground, butthis is of less benefit, because this produces a weaker electrical forceof attraction between the spray and the substrate than if the substratewere electrically grounded or charged to the opposite sign. Electricallygrounding the substrate is the safest mode of operation. Preferably thespray is charged negative relative to electrical ground.

The method of charging the spray is not critical to the practice of theinvention provided the charging method is effective. The precursorcoating composition can be electrically charged by applying highelectrical voltage relative to the substrate and electrical current (1)within the spray gun, by direct contact with electrified walls orinternal electrodes before leaving the orifice; (2) after the sprayemerges from the orifice, by electrical discharge from externalelectrodes located near the orifice and close to the spray; or (3) awayfrom the orifice, by passing the spray through or between electrifiedgrids or arrays of external electrodes before the spray is depositedonto the substrate. Methods (1) and (2), individually or in combination,are preferred. Method (2) is most preferred.

In charging method (1) above, the spray gun must be electricallyinsulating. The high voltage and electrical current is supplied to theadmixed liquid mixture inside the gun by direct contact with an internalsurface that is electrically conducting and electrified. This may bepart of the wall of the flow conduit inside the gun or internalelectrodes that extend into the flow or a combination of electrifiedelements, including the spray nozzle. The contact area must be largeenough to transfer sufficient electrical charge to the admixed liquidmixture as it flows through the gun. This internal charging method hasthe advantage of having no external electrode that could interfere withthe spray. A disadvantage is that if the admixed liquid mixture is notsufficiently electrically insulating, electrical current leakage canoccur through the admixed liquid mixture to a grounded feed supply tankor feed delivery system. This reduces the amount of charge going to thespray. If current leakage is too high, then the feed supply tank andfeed delivery system must be insulated from electrical ground, that is,be charged to high voltage. Current leakage can be measured by measuringthe current flow from the high voltage electrical power supply withoutfluid flow. The current charging the spray is then the differencebetween the current with fluid flow and the current without fluid flow.The leakage current should be small compared to the charging current.

In charging method (2) above, the spray is electrically charged after itemerges from the orifice or in the vicinity of the orifice. The spraygun and spray nozzle must be electrically insulating. The electricalcharge is supplied from external electrode(s) close to the spray tip andadjacent to the spray. Under high electrical voltage, electrical currentis discharged to the spray. The preferred electrodes are one or moremetal wire(s) positioned adjacent to the spray. The electrodes may beeither parallel to the spray or perpendicular to it or any orientationin between such that the electrical current issuing from the sharp pointis favorably directed to the spray. The electrode(s) must be positionedclose enough to the spray, preferably within one centimeter, toeffectively charge the spray without interfering with the flow of thespray. The electrodes may be sharp pointed and may be branched. Forplanar sprays, one or more electrodes are preferably located to theside(s) of the planar spray so that electrical current is discharged tothe face(s) of the spray. For oval sprays, one or more electrodes arelocated adjacent to the spray around the perimeter. The electrode(s) arelocated to effectively charge the spray. One or more auxiliaryelectrodes, which may be at a different voltage than the primaryelectrode(s) or electrically grounded, may be used to modify theelectrical field or current between the primary electrode(s) and thespray. For example, a primary charging electrode may be on one side ofthe spray fan and a grounded insulated auxiliary electrode may by on theopposite side of the spray fan. Charging method (2) has the advantage ofless current leakage through the admixed liquid mixture than chargingmethod (1). Admixed liquid mixtures that are sufficiently conductivemust have the feed supply and feed line insulated from electricalground. In charging method (3) above, the spray is electrically chargedfarther away from the orifice and is more fully dispersed than in method(2). Therefore a larger system or network of external electrodes isusually required in order to effectively charge the spray. Therefore themethod is less safe and less versatile. Also the distance between theelectrodes and spray must be greater to avoid interfering with thespray. Therefore the charge applied to the spray is likely to be lower.But current leakage through the supply lines is eliminated. The spray ispassed through or between electrified grids or arrays of externalelectrodes before the spray is deposited onto the substrate. The spraydroplets are charged by ion bombardment from the electrical currentdischarged into air from the electrodes. The electrified grid may be oneor several wire electrodes that extend across the spray area. Currentcan discharge from along the length of the electrodes. The electrifiedarray may be one or several wire or pointed electrodes positioned aroundthe spray area and which extend close to or into the spray such thatcurrent discharges from the ends of the electrodes.

The present invention can be used with high electrical voltage in therange of about 30 to about 150 kilovolts. Higher electrical voltages arefavored to impart higher electrical charge to the spray to enhanceattraction to the substrate, but the voltage level must be safe for thetype of charging and spray gun used. For safety reasons, the voltage ofhand spray guns is usually restricted to less than 70 kilovolts and theequipment is designed to automatically shut off the voltage when thecurrent exceeds a safe level. Generally for hand spray guns the usefulrange of electrical current is between 20 and 200 microamperes andoptimum results are obtained with coating formulations that have verylow electrical conductivity, that is, very high electrical resistance.For automatic spray guns that are used remotely, higher voltages andelectrical currents can be safely used than for hand spray guns.Therefore the voltage can exceed 70 kilovolts up to 150 kilovolts andthe current can exceed 200 microamperes.

These methods of electrostatic charging are known to those who areskilled in the art of electrostatic spraying.

For electrostatic spraying, the substrate is preferably an electricalconductor such as metal. But substrates that are not conductors orsemiconductors can also be sprayed. Preferably they are pretreated tocreate an electrically conducting surface. For instance, the substratecan be immersed in a special solution to impart conductivity to thesurface.

The method of generating the high electrical voltage and electricalcurrent is not critical to the practice of the current invention.Conventional high voltage electrical power supplies can be used. Thepower supply should have standard safety features that prevent currentor voltage surges. The electrical power supply may be built into thespray gun. Other charging methods may also be used.

The spray pressure used is a function of the precursor coatingcomposition, the supercritical fluid being used, and the viscosity ofthe liquid mixture. The minimum spray pressure is at or slightly belowthe critical pressure of the supercritical fluid. Generally, thepressure will be below 5000 psi. Preferably, the spray pressure is abovethe critical pressure of the supercritical fluid and below 3000 psi. Ifthe supercritical fluid is supercritical carbon dioxide fluid, thepreferred spray pressure is between 1070 psi and 3000 psi. The mostpreferred spray pressure is between 1200 psi and 2500 psi.

The spray temperature used is a function of the precursor coatingcomposition, the supercritical fluid being used, and the concentrationof supercritical fluid in the liquid mixture. The minimum spraytemperature is at or slightly below the critical temperature of thesupercritical fluid. The maximum temperature is the highest temperatureat which the components of the liquid mixture are not significantlythermally degraded during the time that the liquid mixture is at thattemperature.

If the supercritical fluid is supercritical carbon dioxide fluid,because the supercritical fluid escaping from the spray nozzle couldcool to the point of condensing solid carbon dioxide and any ambientwater vapor present due to high humidity in the surrounding sprayenvironment, the spray composition is preferably heated prior toatomization. The minimum spray temperature is about 31° centigrade. Themaximum temperature is determined by the thermal stability of thecomponents in the liquid mixture. The preferred spray temperature isbetween 35° and 90° centigrade. The most preferred temperature isbetween 45° and 75° centigrade. Generally, liquid mixtures with greateramounts of supercritical carbon dioxide fluid require higher spraytemperatures to counteract the greater cooling effect.

If supercritical carbon dioxide fluid is employed as the supercriticalfluid diluent, it preferably should be present in amounts ranging fromabout 10 to about 60 weight percent based upon the total weight of theadmixed coating composition containing the solids fraction, the solventfraction, water and supercritical carbon dioxide, thereby producing amixture having viscosities from about 5 centipoise to about 150centipoise at spray temperature. Most preferably, it is present inamounts ranging from about 20 to about 60 weight percent on the samebasis, thereby producing a mixture having viscosities from about 10centipoise to about 50 centipoise at spray temperature.

The spray temperature may be obtained by heating the liquid mixturebefore it enters the spray gun, by heating the spray gun itself, bycirculating the heated liquid mixture to or through the spray gun tomaintain the spray temperature, or by a combination of methods.Circulating the heated liquid mixture through the spray gun ispreferred, to avoid heat loss and to maintain the desired spraytemperature. Tubing, piping, hoses, and the spray gun are preferablyinsulated or heat traced to prevent heat loss.

The environment into which the admixed coating composition is sprayed isnot critical. However, the pressure therein must be less than thatrequired to maintain the supercritical fluid component of the liquidspray mixture in the supercritical state. Preferably, the admixed liquidcoating composition is sprayed in air under conditions at or nearatmospheric pressure. Other gas environments can also be used, such asair with reduced oxygen content or inert gases such as nitrogen, carbondioxide, helium, argon, xenon, or a mixture. Oxygen or oxygen enrichedair is not desirable, because oxygen enhances the flammability oforganic components in the spray.

Generally, liquid spray droplets are produced which generally have anaverage diameter of one micron or greater. Preferably, these dropletshave average diameters of from about 5 to 1000 microns. More preferably,these droplets have average diameters of from about 10 to about 300microns. Small spray droplets are desirable to vent the supercriticalfluid from the spray droplet before impacting the substrate. Small spraydroplets also give higher quality finishes.

The process of the present invention may be used to apply coatings bythe application of liquid spray to a variety of substrates. Examples ofsuitable substrates include but are not limited to metal, wood, glass,plastic, paper, cloth, ceramic, masonry, stone, cement, asphalt, rubber,and composite materials.

Through the practice of the present invention, films may be applied tosubstrates such that the cured films have thicknesses of from about 0.2to about 6.0 mils. Preferably, the films have thicknesses of from about0.5 to about 2.0 mils, while most preferably, their thicknesses rangefrom about 0.7 to about 1.5 mils.

If curing of the coating composition present upon the coated substrateis required, it may be performed at this point by conventional means,such as allowing for evaporation of the active and/or coupling solvent,application of heat or ultraviolet light, etc.

Compressed gas may be utilized to assist formation of the liquid sprayand/or to modify the shape of the liquid spray that comes from theorifice. The assist gas is typically compressed air at pressures from 5to 80 psi, with low pressures of 5 to 20 psi preferred, but may also beair with reduced oxygen content or inert gases such as compressednitrogen, carbon dioxide, helium, argon, or xenon, or a mixture.Compressed oxygen or oxygen enriched air is not desirable because oxygenenhances the flammability of the organic components in the spray. Theassist gas is directed into the liquid spray as one or morehigh-velocity jets of gas, preferably arranged symmetrically on eachside of the liquid spray to balance each other. The assist gas jets willpreferably come from gas orifices built into the electrostatic spray tipand/or nozzle. The assist gas may also issue from an opening in thespray tip or nozzle that is a concentric annular ring that is around andcentered on the liquid orifice, to produce a hollow-cone high-velocityjet of gas that converges on the liquid spray, but this creates a largerflow of assist gas that is not as desirable. The concentric annular ringmay be divided into segments, to reduce gas flow rate, and it may beelliptical instead of circular, to shape the spray. Preferably, the flowrate and pressure of the assist gas are lower than those used in airspray. The assist gas may be heated to counteract the rapid coolingeffect of the supercritical fluid diluent in the spray. The preferredtemperature of heated assist gas ranges from about 35° to about 90°centigrade. The most preferred temperature ranges from about 45° toabout 75° centigrade.

The precursor coating composition of the present invention may beadmixed with a supercritical fluid and then sprayed onto a substrate bya spray apparatus such as that shown in either FIGS. 3 or 4.

The following illustrates apparatus that may be used to obtain theadmixed coating composition of precursor coating composition andsupercritical fluid and spray it in a batch mode in the practice of thepresent invention. The supercritical fluid illustrated is supercriticalcarbon dioxide fluid.

Table 3 contains a listing of the equipment used in conducting theprocedure described for the batch mode.

                  TABLE 3                                                         ______________________________________                                        Item #   Description                                                          ______________________________________                                         1.      Linde bone-dry-grade liquid carbon dioxide                                    in size K cylinder with eductor tube.                                 2.      Refrigeration heat exchanger.                                         3.      Hoke cylinder #8HD3000, 3.0-liter volume,                                     made of 304 stainless steel, having double                                    end connectors, 1800-psig pressure rating,                                    mounted on scale; carbon dioxide feed tank.                           4.      Circle Seal ™ pressure relief valve                                        P168-344-2000 set at 1800 psig.                                       5.      Vent valve.                                                           6.      Sartorius 16-kilogram scale with 0.1-gram                                     sensitivity.                                                          7.      Hoke cylinder #8HD2250, 2.25-liter volume,                                    made of 304 stainless steel, having double                                    end connectors, 1800-psig pressure rating;                                    pump supply tank.                                                     8.      Zenith single-stream gear pump, model                                         #HLB-5592-30CC, modified by adding a thin                                     Grafoil ™ gasket to improve metal-to-metal                                 seal.                                                                 9.      Zenith gear pump drive model #4204157, with                                   15:1 gear ratio, and pump speed controller                                    #QM-371726F-15-XP, with speed range of 6 to                                   120 revolutions per minute.                                          10.      Drain from circulation loop.                                         11.      Kenics static mixer.                                                 12.      Cooling water heat exchanger.                                        13.      Hoke cylinder #8HD2250, 2.25-liter volume,                                    made of 304 stainless steel, having double                                    end connectors, 1800-psig pressure rating;                                    spray supply tank.                                                   14.      Airless spray gun.                                                   15.      Bonderite ™ 37 polished 24-gauge steel                                     panel, 6-inch by 12-inch size.                                       16.      Vent valve.                                                          17.      Liquid feed valve.                                                   18.      Jerguson high-pressure sight glass series                                     T-30 with window size #6 rated for 2260 psig                                  pressure at 200° F. temperature.                              19.      Grove back-pressure regulator #5-90-W, rated                                  for 2000 psig at 200° F. temperature; dome is                          charged with pressurized nitrogen to desired                                  spray pressure.                                                      20.      Bypass valve.                                                        21.      Pressurized nitrogen to set Grove                                             back-pressure regulator and to purge unit.                           22.      Nitrogen purge valve.                                                23.      Nitrogen vent valve.                                                 24.      Ruska rolling-ball high-pressure viscometer                                   #1602-811-00 with temperature controller and                                  electronic timer.                                                    25.      Pyncnometer, double-valved one-quarter-inch                                   high-pressure tubing.                                                ______________________________________                                    

The apparatus listed in Table 3 above is assembled as shown in theschematic representation contained in FIG. 3. Rigid connections are madewith 1/8-inch diameter high-pressure tubing for gas flows and with1/4-inch diameter high-pressure tubing for liquid flows, using Swagelok™fittings. The spray gun is connected to the tubing by using two Gracoflexible 1/4-inch static-free nylon high-pressure hoses model #061-214with 5000-psi pressure rating. Check valves are used to prevent backflow to the carbon dioxide feed tank (3) and bulk supply tank (1) and tothe nitrogen cylinder (21). The circulation loop and carbon dioxide feedtank are protected from overpressurization by pressure relief valves(4).

The apparatus consists of a circulation loop, a carbon dioxide feedsystem, and a side loop to measure viscosity and density. Thecirculation loop contains a pump supply tank (7), a gear pump (8) toprovide circulation and maintain constant spray pressure, a static mixer(11), a cooler (12) to remove excess heat, a spray supply tank (13), anairless spray gun (14), a sight glass (18), and a pressure regulator(19) to maintain constant spray pressure. The pressure regulator (19) isset by using compressed nitrogen (21) regulated to the desired flowpressure. The carbon dioxide feed system contains a carbon dioxide bulksupply cylinder (1), a refrigeration heat exchanger (2), and a carbondioxide feed tank (3) mounted on an electronic scale (6). The feed andexit lines to the feed tank (3) are coiled so that the force of the tankmoving on the scale does not affect the scale reading. The side loopcontains a viscometer (24) and pyncnometer (25) for measuring theviscosity and density of the admixed coating composition, if desired.

All flow lines and tanks are lined with electrical heat tape and coveredwith insulation to heat the solution to spray temperature. The heat tapeis divided into several circuits that are controlled independently:

Circuit #1 Pressure regulator (19), bypass line (20), sight glass (18),and connecting lines.

Circuit #2 Pump supply tank (7), gear pump (8), and line in circulationloop to carbon dioxide feed point.

Circuit #3 Line in circulation loop from carbon dioxide feed point tocooler (12).

Circuit #4 Spray supply tank (13).

Circuit #5 Line from spray supply tank (13) to flexible hose connectedto spray gun (14).

Circuit #6 Carbon dioxide feed tank (3).

Thermocouples located within the tanks and lines measure temperature.Admixed coating composition temperature is kept uniform around the loopby rapid circulation and by adjusting the heat tapes.

The batch spray unit is filled by the following procedure. The unit isevacuated through the circulation loop vent (16) and a weighed amount ofprecursor coating composition is added through the feed valve (17) withthe gear pump (8) circulating the material at a slow rate through thepressure regulator bypass valve (20). The carbon dioxide feed tank (3)is evacuated through the vent valve (5) and filled with liquid carbondioxide from the carbon dioxide supply cylinder (1). To make filling thefeed tank (3) easier, the carbon dioxide is passed through arefrigeration heat exchanger (2), so that the vapor pressure in the feedtank (3) is lower than the vapor pressure in the supply tank (1). Thedesired mass of carbon dioxide is pressurized into the circulation loopby heating the carbon dioxide feed tank (3) and valving in the desiredamount as read on the balance (6).

The spray pressure is generated by filling the unit with precursorcoating composition and carbon dioxide to the required overall densityand then heating it to the spray temperature. Prior to spraying, thepressure regulator (19) is bypassed (20) and the loop is at a uniformpressure. To prepare for spraying, the bypass (20) is closed so that theflow goes through the pressure regulator (19), which is adjusted to theflow pressure. During spraying, the spray pressure is kept constant bythe gear pump (8) and the pressure regulator (19). The gear pump (8)pumps solution into the spray supply tank (13) from the pump supply tank(7) at a high circulation rate. The pressure regulator (19) dischargesexcess solution back into the pump supply tank (7). The pump supply tank(7) loses inventory and pressure, but the spray supply tank (13) is keptfull and at spray pressure.

The following illustrates apparatus that may be used to obtain theadmixed coating composition of precursor coating composition andsupercritical fluid and spray it in a continuous mode in the practice ofthe present invention. The supercritical fluid illustrated issupercritical carbon dioxide fluid.

Table 4 contains a listing of the equipment used in conducting theprocedure described for the continuous mode.

                  TABLE 4                                                         ______________________________________                                        Item #   Description                                                          ______________________________________                                         1.      Linde bone-dry-grade liquid carbon dioxide                                    in size K cylinder with eductor tube.                                 2.      Refrigeration heat exchanger.                                         3.      Hoke cylinder #8HD3000, 3.0-liter volume,                                     made of 304 stainless steel, having double                                    end connectors, 1800-psig pressure rating.                            4.      Circle Seal ™ pressure relief valve                                        P168-344-2000 set at 1800 psig.                                       5.      Vent valve.                                                           6.      Nitrogen gas supply.                                                  7.      Graco double-acting piston pump model                                         #947-963 with 4-ball design and Teflon ™                                   packings mounted in #5 Hydra-Cat Cylinder                                     Slave Kit #947-943; pump and feed line are                                    refrigeration traced; carbon dioxide pump.                            8.      Graco standard double-acting primary piston                                   pump model #207-865 with Teflon ™ packings;                                coating concentrate pump.                                             9.      Graco Variable Ratio Hydra-Cat ™                                           Proportioning Pump unit model #226-936 with                                   0.9:1 to 4.5:1 ratio range.                                          10.      Graco President air motor model #207-352.                            11.      Utility compressed air at 95 psig supply                                      pressure.                                                            12.      Graco air filter model #106-149.                                     13.      Graco air pressure regulator model #206-197.                         14.      Graco air line oiler model #214-848.                                 15.      Graco pressure relief valve model #208-317                                    set at 3000 psig.                                                    16.      Graco pressure relief valve model #208-317                                    set at 3000 psig.                                                    17.      Graco two-gallon pressure tank model                                          #214-833.                                                            18.      Graco air pressure regulator model #171-937.                         19.      Graco pressure relief valve model #103-437                                    set at 100 psig.                                                     20.      Graco high-pressure fluid heater model                                        #226-816.                                                            21.      Graco high-pressure fluid filter model                                        #218-029.                                                            22.      Graco check valve model #214-037 with                                         Teflon ™ seal.                                                    23.      Graco check valve model #214-037 with                                         Teflon ™ seal.                                                    24.      Graco static mixer model #500-639.                                   25.      Graco high-pressure fluid heater model                                        #226-816.                                                            26.      Graco high-pressure fluid filter model                                        #218-029.                                                            27.      Kenics static mixer.                                                 28.      Graco fluid pressure regulator model                                          #206-661.                                                            29.      Jerguson high-pressure sight glass series                                     T-30                                                                          with window size #6 rated for 2260 psig                                       presure at 200° F. temperature.                               30.      Airless spray gun.                                                   31.      Bonderite ™ 37 polished 24-gauge steel                                     panel, 6-inch by 12-inch size.                                       32.      Zenith single-stream gear pump, model                                         #HLB-5592-30C, modified by adding a thin                                      Grafoil  ™ gasket to improve metal-to-metal                                seal, with pump drive model #4204157,                                         with 15:1 gear ratio, and pump speed                                          controller model #QM-371726F-15-XP, with                                      speed range of 6 to 120 revolutions per                                       minute.                                                              33.      Circle Seal ™ pressure relief valve                                        P168-344-2000 set at 2000 psig.                                      34.      Drain from circulation loop.                                         ______________________________________                                    

The apparatus listed in Table 4 above is assembled as shown in theschematic representation contained in FIG. 4. Rigid connections weremade with Dekuron 1/4-inch diameter, 0.036-inch thick, seamless, welded,type 304 stainless steel hydraulic tubing ASTM A-269 with 5000-psipressure rating, using Swagelok™ fittings. The pressure tank (17) isconnected to the pump (8) using a Graco 3/8-inch static-free nylonhigh-pressure hose model #061-221 with 3000-psi pressure rating. Allother flexible connections are made using Graco 1/4-inch static-freenylon high-pressure hoses model #061-214 with 5000-psi pressure rating.

The precursor coating composition and carbon dioxide are pumped andproportioned by using a Graco Variable Ratio Hydra-Cat™ ProportioningPump unit (9). It proportions two fluids together at a given volumeratio by using two piston pumps (7 and 8) that are slaved together. Thepiston rods for each pump are attached to opposite ends of a shaft thatpivots up and down on a center fulcrum. The volume ratio is varied bysliding pump (7) along the shaft, which changes the stroke length. Thepumps are driven on demand by an air motor (10). Pumping pressure iscontrolled by the air pressure that drives the air motor. The pumps aredouble-acting; they pump on upstroke and downstroke. The primary pump(8) is used to pump the precursor coating composition. It is of standarddesign, having one inlet and one outlet. It fills through a check valveat the bottom and discharges through a check valve at the top. A thirdcheck valve is located in the piston head, which allows liquid to flowfrom the bottom compartment to the top compartment when the piston ismoving downward. This type of pump is designed to be used with low feedpressure, typically below 100 psi. The precursor coating composition issupplied to the primary pump (8) from a two-gallon pressure tank (17).After being pressurized in the pump to spray pressure, the precursorcoating composition is then heated in an electric heater (20) to reduceits viscosity (to aid mixing with carbon dioxide), filtered in a fluidfilter (21) to remove particulates, and fed through a check valve (22)into the mix point with carbon dioxide. The secondary pump (7) on theproportioning pump unit (9) is used to pump the liquid carbon dioxide. Adouble-acting piston pump (7) with a four-check-valve design is usedbecause of the high vapor pressure of carbon dioxide. The pump has aninlet and an outlet on each side of the piston; no flow occurs throughthe piston. The proportion of carbon dioxide pumped into the admixedcoating composition is varied by moving the secondary pump (7) along themoving shaft. Bone-dry-grade liquid carbon dioxide is pumped fromcylinder (1) through refrigeration heat exchanger (2) to secondary pump(7). For measuring the carbon dioxide uptake rate, the carbon dioxide ispumped from Hoke cylinder (3) through heat exchanger (2) to pump (7).The liquid carbon dioxide is refrigerated in heat exchanger (2) in orderto lower the vapor pressure, to prevent cavitation in pump (7). The Hokecylinder (3) is filled from cylinder (1). Air or gaseous carbon dioxidein the cylinder (3) is vented (5) during filling. The Hoke cylinder (3)is mounted on a 16-kilogram Sartorius electronic scale with 0.1-gramsensitivity so that the amount of carbon dioxide in it can be weighed.After being pressurized to spray pressure in pump (7), the liquid carbondioxide is fed unheated through check valve (23) to the mix point withthe precursor coating composition. After the precursor coatingcomposition and carbon dioxide are proportioned together at the mixpoint, the admixed coating composition is mixed in static mixer (24) andpumped on demand into a circulation loop, which circulates the admixedcoating composition at spray pressure and temperature to or through thespray gun (30). The admixed coating composition is heated in an electricheater (25) to obtain the desired spray temperature and filtered in afluid filter (26) to remove particulates. Fluid pressure regulator (28)is installed to lower the spray pressure below the pump pressure, ifdesired, or to help maintain a constant spray pressure. A Jerguson sightglass (29) is used to examine the phase condition of the admixed coatingcomposition. Circulation flow in the circulation loop is obtainedthrough the use of gear pump (32).

The pressure tank (17) is filled with the precursor coating concentrateand pressurized with air to 50 psig. The primary pump (8) is primed byopening a drain valve on the bottom of filter (21) until air was purgedfrom the line.

The carbon dioxide secondary pump (7) is positioned along the pivotingshaft to give the desired percentage of maximum piston displacement. Therefrigeration flow is adjusted to a temperature of -10° C. andcirculated through the refrigeration heat exchanger (2) and therefrigeration tracing on pump (7). The carbon dioxide feed line andcirculation loop are filled with carbon dioxide and vented through valve(34) several times to purge air from the system. Then the valves to themixing point are closed and the carbon dioxide feed line is filled toprime pump (7).

The air pressure regulator (13) is adjusted to supply the air motor (10)with air at the desired pressure to pressurize the feed lines. Thevalves to the mix point are opened and the circulation loop filled withmaterial. With the circulation loop return valve closed, to give plugflow around the circulation loop with no backmixing, material is drainedfrom valve (34) until a uniform composition is obtained. Heater (20) isadjusted to give a feed temperature of 37° C. The circulation heater(25) is adjusted to give the spray temperature. The circulation loopreturn valve is opened and the spray mixture is circulated at a highrate by adjusting the gear pump (32). The carbon dioxide content of theadmixed coating composition is measured by measuring the carbon dioxideuptake rate from Hoke cylinder (3) and the precursor coating compositionuptake rate from pressure tank (17) while spraying through the spraygun. Then the carbon dioxide feed is switched back to supply cylinder(1).

An alternative method of proportioning the precursor coating compositionand supercritical fluid in a continuous mode uses a mass proportionationapparatus instead of the volumetric proportionation apparatusillustrated above. The variable ratio proportioning pump unit (9) withpumps (7) and (8) shown in FIG. 4 is replaced with an apparatus havingthe following elements. For pumping the carbon dioxide, thedouble-acting four-ball piston pump (7) is driven individually on demandby attaching air motor (10) directly to it instead of being driven bythe moving beam. Alternatively, the carbon dioxide can be pumped byusing an air-driven cryogenic pump such as Haskel model DSF-35, which isa single-acting pump that utilizes a three-way cycling spool that isdesigned for pumping liquefied gases under pressure without requiringrefrigeration to avoid cavitation. The pressurized carbon dioxide isthen passed through a pressure regulator, which is used to control thedesired spray pressure, and then through a mass-flow meter, such asMicro Motion model D6, which measures the flow rate of carbon dioxide asit is pumped on demand. For pumping the precursor coating composition,the standard double-acting primary piston pump (8) is replaced with avariable speed gear pump, such as the Zenith gear pump (32) that is usedin the circulation loop. The gear pump pumping rate is controlled by asignal processor that receives the instantaneous carbon dioxide flowrate from the mass flow meter and then controls the gear pump revolutionrate to pump the precursor coating composition at the proper flow rateto give the desired proportion of precursor coating composition andcarbon dioxide in the admixed coating composition. An accumulator, suchas Tobul model 4.7A30-4, may be installed in the circulation loop toincrease the loop capacity and to minimize pressure pulsations in theloop when the spray gun is activated.

EXAMPLES Examples 1-3

In Examples 1-3, the organic solvent of a precursor coating compositioncontaining a water-reducible alkyd as the polymeric coating component isprogressively replaced with larger amounts of water to determine theamount of supercritical carbon dioxide that can be added to each ofthese compositions and to demonstrate that this amount, even after wateraddition, remains substantially the same.

EXAMPLE 1

As a control case containing no water, 135.2 grams of a water-reducibletall oil fatty acid alkyd resin (Cargill 7451, manufactured by Cargill,Inc. which is supplied as a 70% solution in butoxy ethanol) containing94.6 grams of polymer and 40.6 grams of butoxy ethanol; an additional10.8 grams of butoxy ethanol is added to increase the coupling solventused in the composition (total butoxy ethanol coupling solvent=51.4grams); and 16.8 grams of Cymel® 303, a cross-linker manufactured byAmerican Cyanamid Co. are admixed with supercritical carbon dioxide overa pressure range of 1,100 psi to 2,600 psi and a temperature range of35° C. to 55° C.

The maximum amount of supercritical carbon dioxide that can be added tothis "dry" admixture and still provide a clear, single phase is 27% byweight based on the total weight of all of the components of theadmixture.

EXAMPLE 2

In this example, 10% by weight of the organic solvent present in theprecursor composition of Example 1 is replaced with water.

Accordingly, Example 1 is repeated with the exception that 5.0 grams ofdistilled water is substituted for 5.0 grams of butoxy ethanol.

The maximum amount of supercritical carbon dioxide that can now be addedand still provide a clear, single phase is 26% by weight based on thetotal weight of all of the components of the admixture.

EXAMPLE 3

In this example, 20% by weight of the organic solvent present in theprecursor composition of Example 1 is replaced with water.

Accordingly, Example 1 is repeated with the exception that 10.0 grams ofdistilled water is substituted for 10.0 grams of butoxy ethanol.

The maximum amount of supercritical carbon dioxide that the solution cantolerate and avoid phase separation due to saturation with supercriticalcarbon dioxide is 27% by weight based on the total weight of allcomponents. Unlike Examples 1 and 2, substantial clouding of thesolution develops above 12% by weight of supercritical carbon dioxideindicating a phase separation beginning to occur. However, the onset ofsuch clouding does not hinder the spraying performance of thiscomposition.

Examples 4-7

In Examples 4-7, the effects of (1) replacing organic solvent withwater, and (2) adding water to the solvent fraction existing in aprecursor composition containing a water-reducible polyester as thepolymeric coating component are demonstrated.

EXAMPLE 4

As a control case containing no water, 100.0 grams of a water-reducibleoil free polyester resin (Cargill 7203, manufactured by Cargill, inc..which is supplied as a 75% solution in 2-butanol:butoxy ethanol in aratio of 2.97:1) containing 75.0 grams of polymer, 18.7 grams of2-butanol and 6.3 grams of butoxy ethanol; an additional 25.0 grams ofbutoxy ethanol is added to increase the coupling solvent used in thecomposition (total butoxy ethanol coupling solvent=31.3 grams); and 25.0grams of Cymel® 303 are admixed with supercritical carbon dioxide overthe same temperature and pressure ranges set forth in Example 1.

The maximum amount of supercritical carbon dioxide that the solution cantolerate, remain clear, and avoid phase separation due to saturationwith supercritical carbon dioxide is 31 wt % based on the total weightof all components.

EXAMPLE 5

As yet another control case containing no water, Example 4 is repeatedwith the exception that the total amount of butoxy ethanol is reduced by5.0 grams. The maximum amount of supercritical carbon dioxide that thesolution can tolerate, remain clear, and avoid phase separation due tosaturation with supercritical carbon dioxide is 30% by weight based ontotal weight of all components.

EXAMPLE 6

In this example, 10% by weight of the organic solvent present in theprecursor composition of Example 4 is replaced with water, which exampleis also equivalent to adding 11% by weight of water based on the totalweight of organic solvent to the precursor composition of Example 5.

Accordingly, Example 4 is repeated with the exception that 5.0 grams ofdistilled water is substituted for 5.0 grams of butoxy ethanol. Example5 is also repeated except that 5.0 grams of water is added to thecomposition.

In each instance, the maximum amount of supercritical carbon dioxidethat the solution can tolerate, remain clear, and avoid phase separationdue to saturation with supercritical carbon dioxide is 29% by weightbased on the total weight of all of the components.

In the example, 20% by weight of the organic solvent present in theprecursor composition of Example 4 is replaced with water.

Accordingly, Example 4 is repeated with the exception that 10.0 grams ofdistilled water is substituted for 10.0 grams of butoxy ethanol. Themaximum amount of supercritical carbon dioxide that the solution cantolerate, remain clear, and avoid phase separation due to saturationwith supercritical carbon dioxide is 25%. Unlike Examples 4-6,substantial clouding of the solution develops above 15% by weight ofsupercritical carbon dioxide indicating phase separation beginning tooccur. However, the onset of such clouding does not hinder the sprayingperformance of this composition.

Examples 8-11

In Examples 8-11, the organic solvent of a precursor coating compositioncontaining a conventional polyester resin as the polymeric component isprogressively replaced with larger amounts of water to determine theamount of supercritical carbon dioxide that can be added to each ofthese compositions and to demonstrate that this amount, even after wateraddition, remains substantially the same.

EXAMPLE 8

As a control case containing no water, 75.0 grams of an oil freepolyester resin (Cargill 5780, manufactured by Cargill, Inc. in solventfree form); 50.0 grams of butoxy ethanol; and 25.0 grams of Cymel® 303are admixed with supercritical carbon dioxide over the same temperatureand pressure ranges set forth in Example 1.

The maximum amount of supercritical carbon dioxide that the solution cantolerate, remain clear, and avoid phase separation due to saturationwith supercritical carbon dioxide is 32 wt % based on the total weightof all components.

EXAMPLE 9

In this example, 10% by weight of the organic solvent present in theprecursor composition of Example 8 is replaced with water.

Accordingly, Example 8 is repeated with the exception that 5.0 grams ofdistilled water is substituted for 5.0 grams of butoxy ethanol.

The maximum amount of supercritical carbon dioxide that can now be addedand still provide a clear, single phase is 32% by weight based on thetotal weight of all of the components of the admixture.

EXAMPLE 10

In this example, 20% by weight of the organic solvent present in theprecursor composition of Example 8 is replaced with water.

Accordingly, Example 8 is repeated with the exception that 10.0 grams ofdistilled water is substituted for 10.0 grams of butoxy ethanol.

The maximum amount of supercritical carbon dioxide that the solution cantolerate, remain clear, and avoid phase separation due to saturationwith supercritical carbon dioxide is 28 wt % based on the total weightof all components. Moderate clouding of the solution occurs above 23% byweight of supercritical carbon dioxide. This indicates that a very smallamount of phase separation is occurring.

EXAMPLE 11

In this example, 30% by weight of the organic solvent present in theprecursor composition of Example 8 is replaced with water.

Accordingly, Example 8 is repeated with the exception that 15.0 grams ofdistilled water is substituted for 15.0 grams of butoxy ethanol.

The maximum amount of supercritical carbon dioxide that the solution cantolerate and avoid phase separation due to saturation with supercriticalcarbon dioxide is 26% by weight based on the total weight of allcomponents. Clouding of the solution develops upon the first addition ofsupercritical carbon dioxide indicating that an undesirable phaseseparation is occurring.

Examples 12-14

Examples 12-14 illustrate the desirability of using a coupling solvent(butoxy ethanol) in the precursor composition when water is present.

EXAMPLE 12

This example shows the effect of replacing 25 wt % of the couplingsolvent in Example 10 with a non-coupling active solvent, ethyl3-ethoxypropionate.

Example 10 is repeated with the exception that 10.0 grams of ethyl3-ethoxypropionate is substituted for 10.0 grams of butoxy ethanol. Themaximum amount of supercritical carbon dioxide that the solution cantolerate and avoid phase separation due to saturation with supercriticalcarbon dioxide is 28% by weight based on the total weight of allcomponents. Substantial clouding of the solution develops above 26% byweight of supercritical carbon dioxide indicating that phase separationis beginning to occur.

EXAMPLE 13

This example shows the effect of replacing 50 wt % of the couplingsolvent in Example 10 with a non-coupling active solvent, ethyl3-ethoxypropionate.

Example 10 is repeated with the exception that 20.0 grams of ethyl3-ethoxypropionate is substituted for 20.0 grams of butoxy ethanol. Themaximum amount of supercritical carbon dioxide that the solution cantolerate and avoid phase separation due to saturation with supercriticalcarbon dioxide is 25% by weight based on the total weight of allcomponents. Substantial clouding of the solution develops upon the firstaddition of supercritical carbon dioxide indicating an undesirable phaseseparation is occurring.

EXAMPLE 14

This example shows the effect of replacing all of the coupling solventin Example 10 with a non-coupling active solvent, ethyl3-ethoxypropionate.

Example 10 is repeated with the exception that 40.0 grams of ethyl3-ethoxypropionate is substituted for all 40.0 grams of butoxy ethanol.A two phase mixture forms before any supercritical carbon dioxide isadded. It is known that ethyl -ethoxypropionate unlike butoxy ethanol isimmiscible in water; however, it is a solvent structurally related to2-ethoxyethyl acetate which has significant water solubility. Thus ethyl3-ethoxypropionate is polar enough to be close to the boundaryseparating water miscible and immiscible solvents. The maximum amount ofsupercritical carbon dioxide that the two phase mixture can tolerate andavoid additional phase separation due to saturation with supercriticalcarbon dioxide is 26% by weight based on the weight of all components.

The above examples show that despite the low water solubility of carbondioxide, it is possible to replace some of the organic solvent withwater in coatings formulations using both water-reducible andconventional resins and obtain mixtures which are suitable for admixturewith supercritical fluids, such as supercritical carbon dioxide.

Both advantages of water addition are feasible. Water can be added to acoating formulation to reduce the viscosity while maintaining the amountof organic solvent constant (e.g., Examples 4 and 5). Moreover, watercan also be substituted for organic solvent to lower the amount ofoverall solvent present in a coating while still maintaining theviscosity level (e.g., Examples 4 and 6).

A sufficient amount of a coupling solvent is desired to couple the waterinto the organic coating, but once there is a sufficient amount of suchcoupling solvent, the water can be used to further dilute the coatingfor viscosity reduction to enable spraying.

What is claimed is:
 1. A precursor coating composition suitable foradmixture with at least one supercritical fluid which admixture is thento be sprayed comprising a liquid mixture of:(a) a solids fractioncontaining at least one polymeric compound capable of forming a coatingon a substrate; and (b) a solvent fraction containing at least onecoupling solvent in which said at least one polymeric compound is atleast partially soluble and which is at least partially miscible withwater, said solvent fraction having a composition containing a weight %solvent distribution and relative evaporation rates of:

    ______________________________________                                        Wt % of Total Solvent Fraction                                                                     RER                                                      ______________________________________                                        30-100%              <50                                                      0-70%                 50-100                                                  0-40%                101-250                                                  <10%                 >250                                                     ______________________________________                                    

and said coupling solvent being selected from the group consisting ofethylene glycol ethers, propylene glycol ethers, lactams, cyclic ureas,and mixtures thereof; and (c) water, which is present in an amount ofless than about 30% by weight based on the weight of the solventfraction; said liquid mixture having:(i) a viscosity of less than about6,000 centipoise and having less than about 400 grams of the solventfraction per liter of mixture; (ii) a solubility with at least onesupercritical fluid, above the critical temperature and pressure of thesupercritical fluid, of greater than 5% by weight of supercritical fluidin said mixture; (iii) a viscosity of less than about 300 centipoisewhen admixed with a sufficient amount of the at least one supercriticalfluid, above the critical temperature and pressure of the supercriticalfluid, so as to render the mixture suitable for spray application; and(iv) a solubility with the supercritical fluid in the non-supercriticalstate, at 25° C. and one atmosphere absolute pressure of said fluid, ofless than about 0.8% by weight of fluid in said mixture.
 2. Theprecursor composition of claim 1, wherein the coupling solvent isselected from the group consisting of butoxy ethanol, propoxy ethanol,hexoxy ethanol, isoproproxy 2-propanol, butoxy 2-propanol, propoxy2-propanol, tertiary butoxy 2-propanol, ethoxy ethanol, butoxy ethoxyethanol, propoxy ethoxy ethanol, hexoxy ethoxy ethanol, methoxy ethanol,methoxy 2-propanol, ethoxy ethoxy ethanol, n-methyl-2-pyrrolidone,dimethyl ethylene urea, and mixtures thereof.
 3. A precursor coatingcomposition suitable for admixture with at least one supercritical fluidwhich admixture is then to be sprayed comprising a liquid mixture of:(a)a solids fraction containing at least one polymeric compound capable offorming a coating on a substrate; and (b) a solvent fraction containingat least one coupling solvent in which said at least one polymericcompound is at least partially soluble and which is at least partiallymiscible with water, and containing at least one active solvent in whichsaid polymeric compound is at least partially soluble and which is atleast partially miscible with the supercritical fluid, said solventfraction having a composition containing a weight % solvent distributionand relative evaporation rates of:

    ______________________________________                                        Wt % of Total Solvent Fraction                                                                     RER                                                      ______________________________________                                        30-100%              <50                                                      0-70%                 50-100                                                  0-40%                101-250                                                  <10%                 >250                                                     ______________________________________                                    

and said coupling solvent being selected from the group consisting ofethylene glycol ethers, propylene glycol ethers, lactams, cyclic ureas,and mixtures thereof; and (c) water, which is present in an amount ofless than about 30% by weight based on the weight of the solventfraction; said liquid mixture having:(i) a viscosity of less than about6000 centipoise and having less than about 400 grams of the solventfraction per liter of mixture; (ii) a solubility with supercriticalfluid, above the critical temperature and pressure of the supercriticalfluid, of greater than 5% by weight of supercritical fluid in saidmixture; (iii) a viscosity of less than about 300 centipoise whenadmixed with a sufficient amount of the at least one supercriticalfluid, above the critical temperature and pressure of the supercriticalfluid, so as to render the mixture suitable for spray application; and(iv) a solubility with the supercritical fluid in the non-supercriticalstate, at 25° C. and one atmosphere absolute pressure of said fluid, ofless than about 0.8% by weight of fluid in said mixture.